President Joe Biden, under pressure to do more to aid Ukraine in its war against Russia, has warned that any military escalation by the West against Moscow would be catastrophic.

On Friday, reporters peppered Biden with questions about what type of Russian escalation would trigger a military counterresponse by the U.S. during a press conference where he called on Congress to enact new trade sanctions against the country. The president said Washington or NATO would spark "World War III" if it engaged in a military confrontation with Russia.

"We will defend every single inch of NATO territory with the full might of the united and galvanized NATO," Biden said, repeating a line he has used previously in response to similar questions. "We will not fight a war against Russia in Ukraine. Direct conflict between NATO and Russia is World War III, something we must strive to prevent."

Biden’s unequivocal opposition to a U.S. or NATO intervention in Ukraine has been a position he has maintained since December when he ruled out any U.S. forces going to the country to face a Russian invasion. It follows an effort by his administration to balance tough actions that raise the costs on Russia’s economy without feeding President Vladimir Putin’s fears that Washington was looking to overthrow him.

Ukraine’s President Voldomyr Zelensky has repeated his requests for a no-fly zone maintained by NATO over his country. Zelensky, who has remained in the Ukrainian capital Kyiv as Russian forces surround the city, has vented his frustrations about what he sees as the West’s unwillingness to confront Putin. On Thursday, the Ukrainian leader waved off Putin’s previous threats of nuclear escalation as a “bluff.”

But Biden’s concerns have been reinforced by his intelligence community. During testimony before Congress on Wednesday, the chiefs of major intelligence agencies like the CIA cautioned that a nuclear strike by Putin could not be easily dismissed.

Several Republicans have lampooned Biden for not granting Ukraine’s request for a no-fly zone. However, even more are opposed to the move for fear of nuclear escalation, including Russia hawks like Sen. Marco Rubio, R-Fla.

"I think people need to understand what a no-fly zone means ... it's not some rule you pass that everybody has to oblige by," Rubio told ABC News on Sunday. "It's the willingness to shoot down the aircrafts of the Russian Federation, which is basically the beginning of World War III."

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