Volodymyr Zelenskyy has warned if Ukraine is defeated in its war against Russia, it could eventually lead to World War III.
He also renewed calls for continued US support, 18 months after Russia launched its full-scale invasion of Ukraine.
The Ukrainian president told CBS' 60 Minutes program on Monday that American funding is serving an important cause.
"If Ukraine falls, what will happen in 10 years?" he said.
"If [Russian troops] reach Poland, what's next? A third world war?"
When asked if Ukraine would need extra cash to bolster its armed forces, Zelenskyy replied: "I don't have an answer."
The need for further financial support depends on whether the nations of the world are determined to beat Russian leader Vladimir Putin, he said.
"We can't change Putin."
He said Russian society had lost the world's respect.
"They elected him and re-elected him and raised a second Hitler. They did this. We cannot go back in time. But we can stop it here."
Zelenskyy expects Russia's nuclear weapons threats to ramp up towards November next year, as the US holds its 2024 presidential elections.
"He is waiting for the United States to become less stable. He thinks that's going to happen during the US election."
Ukraine's leader will travel to Washington to meet US President Joe Biden this week - part of a wider trip for the United Nations General Assembly.
It comes as Biden tries to get Congress to approve a US$24 billion support package for Kyiv - a package Republicans don't agree with.
House Speaker Kevin McCarthy, a Republican, told US media he expected to meet with Zelenskyy too.
The US Congress has approved US$113 billion worth of support for Ukraine so far.
More than half, or US$70 billion, was for security and intelligence, the New York Times reported.
