Ukrainian President Volodymyr Zelenskyy warned that World War III may have already started when Russian troops invaded his country and that "civilization is at stake" during an American TV interview hours after addressing Congress Wednesday.

"Nobody knows whether it may have already started," he told NBC News’ Lester Holt. "And what is the possibility of this war if Ukraine will fall, in case Ukraine will? It's very hard to say…We've seen this 80 years ago, when the Second World War had started...nobody would be able to predict when the full-scale war would start."

Vladimir Putin ordered Russian troops to invade Ukraine on Feb. 24 – launching what analysts initially predicted would be a quick war to oust Zelenskyy’s government and replace it with Kremlin-friendly puppets.

In this image from video provided by the House Television, Ukrainian President Volodymyr Zelenskyy speaks from Kyiv, Ukraine, as he addresses virtually addresses Congress on Wednesday, March 16, 2022. (House Television via AP)

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Western leaders, including at the top of the U.S. and NATO, have publicly stated they would not get militarily involved on Ukrainian soil out of fear it could provoke Russia to escalate the conflict and start World War III.

Meanwhile, Ukraine’s defense forces refused to let the Russians walk over them, putting up fierce resistance and slowing the invaders to a halt three weeks into the invasion. Russia’s air force, larger, newer and superior by most metrics, has failed to achieve air superiority, and Ukrainian defense forces claim to have taken out multiple Russian generals and more than 12,000 troops.

The death toll could not be independently confirmed, and Russia said earlier this month it had lost only a small fraction of that total.

Zelenskyy and other Ukrainian officials for weeks have been warning that an attack on their country is an attack and Europe, appealing for more help and suggesting that Putin’s ambitions extend beyond conquering his smaller neighbor.

As Russian forces have also failed to take Kyiv, the Ukrainian capital, they’re stepping up attacks on civilian targets, including a maternity hospital and a makeshift shelter holding an estimated 1,000 civilians in a theater, both in the besieged city of Mariupol.

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Earlier Wednesday, Zelenskyy addressed both chambers of the U.S. Congress, asking for increased military aid and reiterating his call for a no-fly zone.

"Remember Pearl Harbor, [the] terrible morning of Dec. 7, 1941 when your sky was black from planes attacking you," he said, through a translator. "Remember Sept. 11, a terrible day in 2001 when evil tried to turn U.S. cities…into a battlefield. Innocent people were attacked from the air….Our country experiences the same every day, right now."

Image grab from footage released by Ukraine Armed Forces on Thursday March 9, 2022 allegedly shows aftermath of Russia army bombardment on a children hospital in Mariupol, southeastern Ukraine, causing enormous destructions. The Ukraine Armed Forces does not indicate casualties of the strikes. Ukraine President Volodymyr Zelensky confirms the strike of the maternity hospital in Mariupol, saying in his online post (EYEPRESS via Reuters Connect)

Speaker of the House Nancy Pelosi, D-Calif., introduces Ukrainian President Volodymyr Zelenskyy to speak to the U.S. Congress by video at the Capitol in Washington, Wednesday, March 16, 2022. (AP Photo/J. Scott Applewhite, Pool)

People look at a burning apartment building in a yard after shelling in Mariupol, Ukraine, Sunday, March 13, 2022. (AP Photo/Evgeniy Maloletka))

President Biden announced another $1 billion in defensive aid to Ukraine Wednesday and went as far as to call Putin a "war criminal," but his administration has resisted Zelenskyy’s calls for a no-fly zone over Ukrainian skies, which NATO officials say would put the U.S.-led alliance in direct military conflict with Russia.

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Fox News’ Michael Lee and Greg Norman contributed to this report.