Vladimir Putin poses a "real" threat of using tactical nuclear weapons, Joe Biden has said.
The US President has previously denounced Russia moving nuclear arms to Belarus as "absolutely irresponsible".
Speaking to donors an event in California on Monday night, Mr Biden said he was "worried" about Putin using the catastrophic weapons.
"When I was out here about two years ago saying I worried about the Colorado river drying up, everybody looked at me like I was crazy," he said.
"They looked at me like when I said I worry about Putin using tactical nuclear weapons. It's real."
Belarusian President Alexander Lukashenko said his country has started taking delivery of Russian tactical nuclear weapons last week.
Some are believed to be three times more powerful than the atomic bombs the US dropped on Hiroshima and Nagasaki in 1945.
The deployment is the Kremlin's first move of shorter-range, less powerful nuclear weapons that could be used on the battlefield outside Russia since the fall of the Soviet Union.
Meanwhile Moscow launched attacks across Ukraine in the early hours of Tuesday, hitting Kyiv and the western city of Lviv, close to the border with Poland.
The Ukrainian capital's military administration said Moscow had launched a "massive air attack" on Tuesday morning and warned residents to find shelter.
The city's air defences detected and destroyed about two-dozen Iranian-made Shahed drones, Kyiv authorities said.
It is believed to be the second drone attack on the Kyiv this month.
The capital saw a "restless and noisy night" but the situation is "stable", the head of the city council, Anatoly Kurtev, said on Telegram.
No casualties have been reported and there was no damage to residential buildings or utility systems, he added.
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