Russia’s Defense Ministry on Sunday warned that any country that offers the use of its airfields to Ukraine’s military for attacks on Russian assets could be considered as having entered the conflict.

“The use of the airfield networks of these countries to base Ukrainian military aircraft and their subsequent use against the Russian armed forces may be regarded as the involvement of these states in an armed conflict,” Russian Defense Ministry spokesman Igor Konashenkov told the Interfax news agency on Sunday evening local time.

Konashenkov said Russian officials are aware of “Ukrainian combat plans which earlier flew to Romania and other neighboring countries,” without elaborating.

Since the conflict started on Feb. 24, the United States, its NATO allies, and other European nations have flooded Ukraine with missiles, anti-tank missiles, ammunition, weapons, and other supplies. There have also been reports claiming that European and NATO countries are working to sent fighter jets to Ukraine.

A report from the Financial Times, meanwhile, quoted an anonymous White House official as saying the United States is working with Polish officials to send the jets to Ukraine’s military, but Poland’s government disputed those reports as false.

“FAKE NEWS! Unfortunately you are spreading misinformation with quotation from 27/02/22. Poland won’t send its fighter jets to #Ukraine as well as allow to use its airports. We significantly help in many other areas,” the Polish prime minister’s office wrote on Sunday morning in a Twitter post to the pro-Kyiv news outlet NEXTA, which repeated the claim that Poland and the United States are trying to get fighter planes to Kyiv.

Ukrainian President Volodymyr Zelensky has repeatedly asked the United States and NATO to provide jets or enforce a no-fly zone, urging American members of Congress on Friday again to do so in a Zoom call.

But NATO’s Secretary-General, Jens Stoltenberg, and top White House officials have said that a no-fly zone is not being considered over Ukraine, which is not a member of NATO, because it would entail having U.S. or NATO planes shooting down Russian military aircraft or attacking assets in Ukraine and inside Russia. Stoltenberg warned late last week that the move would lead to a sharp escalation in the conflict with Russia, which has perhaps the largest arsenal of nuclear weapons in the world.

Despite the risks, some members of Congress, including outgoing Rep. Adam Kinzinger (R-Ill.), have called for a no-fly zone to be enforced over the Eastern European nation.

On Sunday morning, Sen. Marco Rubio (R-Fla.) told ABC News that a no-fly zone over Ukraine would lead to another world war.

“A no-fly zone has become a catchphrase. I’m not sure a lot of people fully understand what that means,” Rubio told ABC News. “That means flying AWACS 24 hours a day, that means the willingness to shoot down and engage Russian airplanes in the sky. That means, frankly, you can’t put those planes up there unless they’re willing to knock out the anti-aircraft systems that the Russians have deployed in, and not just in Ukraine, but Russia and also in Belarus,” Rubio said.