President Vladimir V. Putin of Russia has warned that if F-16 fighter jets supplied to Ukraine by its Western allies operated from airfields in other countries, the bases would be “legitimate targets” for attack.

In a speech to Russian Air Force pilots late Wednesday, however, Mr. Putin rejected suggestions from some Western leaders that Russia is planning to invade NATO countries as “complete nonsense.”

The threat that Russia might move against other countries has become one of the main arguments used by the Ukrainian government and its supporters to try to persuade the U.S. to dispatch more military aid to the country.

The Ukrainian president, Volodymyr Zelensky, said again in an interview with CBS News published on Thursday that war “can come to Europe, and to the United States of America.”

“It can come very quickly to Europe,” said Mr. Zelensky.

At the United Nations on Thursday, Russia vetoed a resolution that would have extended U.N. oversight of sanctions tied to North Korea’s nuclear program, prompting accusations from Western nations that it was acting to keep the pipeline of weaponry open from the North Koreans for use in Ukraine.

The Russian vote — a reversal from its past support for the monitoring program — drew condemnation from world leaders. In the United States, the White House national security spokesman, John Kirby, called it a “reckless action” that made clear the growing ties between Pyongyang and Moscow.

The vote came a day after Mr. Putin visited the provincial town of Torzhok in the Tver region north of Moscow, where he inspected a training center for Russian military pilots.

Speaking with the pilots, Mr. Putin said that Russia spent only a fraction on its military of what the United States does. “Considering this difference,” he said, concerns that Moscow would fight NATO are “nonsense.”

Mr. Putin also reiterated his argument that NATO’s enlargement in the wake of the Soviet collapse was the primary reason for his decision to attack Ukraine in 2022.

“Did we cross the ocean and approach U.S. borders?” he asked. “No, they are the ones who are closing in on us, and they have reached our borders.”

But Mr. Putin did leave open the door to an expanded battlefield when it came to the decision by Ukraine’s allies to equip it with American-made F-16 fighter jets.

“F16 aircraft can also carry nuclear weapons, and we will have to heed this while organizing our combat operations,” said Mr. Putin.

At the United Nations on Thursday, Russia vetoed a resolution that would have extended U.N. oversight of sanctions tied to North Korea’s nuclear program, prompting accusations from Western nations that it was acting to keep the pipeline of weaponry open from the North Koreans for use in Ukraine.

Ukraine has also figured large in the Kremlin’s response to the terrorist attack that killed at least 143 people last week at a concert hall outside Moscow.

Since shortly after the attack, Mr. Putin and other Russian officials, without evidence, have depicted Ukraine as the likely mastermind behind it. Mr. Putin eventually acknowledged that the assault had been “perpetrated by radical Islamists,” but stuck with the contention that Ukraine could have ordered it.

On Thursday, continuing in that vein, Russia’s investigative committee, the country’s equivalent to the F.B.I., said in a statement that it had “obtained evidence” that the attackers were “connected with Ukrainian nationalists,” without specifying what that proof was.

Investigators said that they had “confirmed data that the perpetrators of the terrorist attack received significant amounts of money and cryptocurrency from Ukraine, which were used in preparing the crime.” They also said that another person had been arrested in connection with the attack and would soon appear in court.

The Ukrainian government has denied any involvement in the assault. The United States and other Western governments have said repeatedly that the Islamic State — which itself has issued two claims of responsibility — was behind the assault and that Ukraine was not involved.

With 74 victims of the concert hall attack still hospitalized, Mr. Putin has yet to visit them or to go to the site of the attack. Dmitri S. Peskov, the Kremlin spokesman said on Thursday that Mr. Putin had been heavily involved in the rescue effort on the night of the attack, and that a visit to the site of the attack would hinder their work.

Emin Agalarov, an executive with the owner of the concert hall, the Crocus Group, on Thursday defended how his staff had handled the attack and its aftermath, including a fire in the hall, and the building’s complied with fire safety and security requirements.

Mr. Agalarov that security guards barred by law from carrying firearms by law. Speaking on state television, Mr. Agalarov said that fire and other emergency systems worked normally and allowed thousands of people to evacuate the building.

The United States and China are locked in a new race, in space and on Earth, over a fundamental resource: time itself.

And the United States is losing.

Global positioning satellites serve as clocks in the sky, and their signals have become fundamental to the global economy — as essential for telecommunications, 911 services and financial exchanges as they are for drivers and lost pedestrians.

But those services are increasingly vulnerable as space is rapidly militarized and satellite signals are attacked on Earth.

Yet, unlike China, the United States does not have a Plan B for civilians should those signals get knocked out in space or on land.

The risks may seem as remote as science fiction. But just last month, the United States said that Russia may deploy a nuclear weapon into space, refocusing attention on satellites’ vulnerability. And John E. Hyten, an Air Force general who also served as vice chairman of the Joint Chiefs of Staff, and who is now retired, once called some satellites “big, fat, juicy targets.”

Tangible threats have been growing for years.

Russia, China, India and the United States have tested antisatellite missiles, and several major world powers have developed technology meant to disrupt signals in space. One Chinese satellite has a robotic arm that could destroy or move other satellites.

Other attacks are occurring on Earth. Russian hackers targeted a satellite system’s ground infrastructure in Ukraine, cutting off internet at the start of the war there. Attacks like jamming, which drowns out satellite signals, and spoofing, which sends misleading data, are increasing, diverting flights and confounding pilots far from battlefields.

If the world were to lose its connection to those satellites, the economic losses would amount to billions of dollars a day.

Despite recognizing the risks, the United States is years from having a reliable alternative source for time and navigation for civilian use if GPS signals are out or interrupted, documents show and experts say. The Transportation Department, which leads civilian projects for timing and navigation, disputed this, but did not provide answers to follow-up questions.

A 2010 plan by the Obama administration, which experts had hoped would create a backup to satellites, never took off. A decade later, President Donald J. Trump issued an executive order that said that the disruption or manipulation of satellite signals posed a threat to national security. But he did not suggest an alternative or propose funding to protect infrastructure.

The Biden administration is soliciting bids from private companies, hoping they will offer technical solutions. But it could take years for those technologies to be widely adopted.

Where the United States is lagging, China is moving ahead, erecting what it says will be the largest, most advanced and most precise timing system in the world.

It is building hundreds of timing stations on land and laying 12,000 miles of fiber-optic cables underground, according to planning documents, state media and academic papers. That infrastructure can provide time and navigation services without relying on signals from Beidou, China’s alternative to GPS. It also plans to launch more satellites as backup sources of signals.

“We should seize this strategic opportunity, putting all our efforts into building up capabilities covering all domains — underwater, on the ground, in the air, in space and deep space — as soon as possible,” researchers from the China Aerospace Science and Industry Corporation, a state-owned conglomerate, wrote in a paper last year.

China retained and upgraded a World War II-era system, known as Loran, that uses radio towers to beam time signals across long distances. An enhanced version provides signals to the eastern and central parts of the country, extending offshore to Taiwan and parts of Japan. Construction is underway to expand the system west.

Russia, too, has a long-range Loran system that remains in use. South Korea has upgraded its system to counter radio interference from North Korea.

The United States, though, decommissioned its Loran system in 2010, with President Barack Obama calling it “obsolete technology.” There was no plan to replace it.

In January, the government and private companies tested an enhanced version of Loran on U.S. Coast Guard towers. But companies showed no interest in running the system without government help, so the Coast Guard plans to dispose of all eight transmission sites.

“The Chinese did what we in America said we would do,” said Dana Goward, the president of the Resilient Navigation and Timing Foundation in Virginia. “They are resolutely on a path to be independent of space.”

What Is the United States Doing?

Since Mr. Trump’s executive order, about a dozen companies have proposed options, including launching new satellites, setting up fiber optic timing systems or restarting an enhanced version of Loran. But few products have come to market.

A private firm, Satelles, working with the U.S. National Institute of Standards and Technology in Colorado, has developed an alternative source for time using satellites that were already orbiting about 485 miles above Earth.

N.I.S.T. scientists say the signals are a thousand times stronger than those from GPS satellites, which orbit more than 12,000 miles above Earth. That makes them harder to jam or spoof. And because low-Earth-orbit satellites are smaller and more dispersed, they are less vulnerable than GPS satellites to an attack in space.

The satellites obtain time from stations around the world, including the N.I.S.T. facility in Colorado and an Italian research center outside Milan, according to Satelles’s chief executive, Michael O’Connor.

China has similar plans to upgrade its space-time system by 2035. It will launch satellites to augment the Beidou system, and the country plans to launch nearly 13,000 satellites into low-Earth orbit.

China says its investments are partly motivated by concerns about an American attack in space. Researchers from China’s Academy of Military Sciences have said that the United States is “striving all-out” to build its space cyberwarfare abilities, especially after the war in Ukraine brought “a deeper appreciation of the critical nature of space cybersecurity.”

The United States has increased its spending on space defense, but Space Force, a branch of the military, did not answer specific questions about the country’s antisatellite abilities. It said it was building systems to secure the nation’s interests as “space becomes an increasingly congested and contested domain.”

Separate from civilian use, the military is developing GPS backup options for its own use, including for weapons like precision-guided missiles. Most of the technologies are classified, but one solution is a signal called M-code, which Space Force says will resist jamming and perform better in war than civilian GPS. It has been plagued by repeated delays, however.

The military is also developing a positioning, timing and navigation service to be distributed by low-Earth-orbit satellites.

Other countermeasures look to the past. The U.S. Naval Academy resumed teaching sailors to navigate by the stars.

What Happens if the U.S. Doesn’t Find a Solution?

Satellite systems — America’s GPS, China’s Beidou, Europe’s Galileo and Russia’s Glonass — are the important sources of time, and time is the cornerstone of most methods of navigation.

In the American GPS system, for example, each satellite carries atomic clocks and transmits radio signals with information about its location and the precise time. When a cellphone receiver picks up signals from four satellites, it calculates its own location based on how long it took for those signals to arrive.

Cars, ships and navigation systems on board aircraft all operate the same way.

Other infrastructure relies on satellites, too. Telecom companies use precise time to synchronize their networks. Power companies need time from satellites to monitor the state of the grid and to quickly identify and investigate failures. Financial exchanges use it to keep track of orders. Emergency services use it to locate people in need. Farmers use it to plant crops with precision.

A world without satellite signals is a world that is nearly blind. Ambulances will be delayed on perpetually congested roads. Cellphone calls will drop. Ships may get lost. Power outages may last longer. Food can cost more. Getting around will be much harder.

Yet, some critical civilian systems were designed with a flawed assumption that satellite signals would always be available, according to the U.S. Cybersecurity and Infrastructure Security Agency.

That reliance can have dire consequences. A recent report from Britain showed that a weeklong outage of all satellite signals would cost its economy nearly $9.7 billion. An earlier report put the toll on the U.S. economy at $1 billion a day, but that estimate is five years old.

“It’s like oxygen, you don’t know that you have it until it’s gone,” Adm. Thad W. Allen, a former commandant of the U.S. Coast Guard who leads a national advisory board for space-based positioning, navigation and timing, said last year.

For now, mutually assured losses deter major attacks. Satellite signals are transmitted on a narrow radio band, which makes it difficult for one nation to jam another’s satellite signals without shutting off its own services.

Having GPS for free for 50 years has “gotten everybody addicted,” according to Mr. Goward from the Resilient Navigation and Timing Foundation. The government has not done enough to make alternatives available to the public, he said.

“It’s only admiring the problem,” he said, “not solving the problem.”