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.