Joe Biden says that Nato would respond “in kind” if Vladimir Putin used chemical weapons in Ukraine.
The president spoke after a day of summits in Belgium with Nato allies, the European Union and G7 partners.
Mr Biden was asked if chemical weapons were used in Ukraine, “would that trigger a military response from Nato”.
Mr Biden replied: “It would trigger a response in kind, whether or not, you’re asking whether Nato would cross, we’d make that decision at the time.”
During the meetings Mr Biden also said he supported expelling Russia from the G20 economic group, and that if it is not possible Ukraine should be able to attend meetings.
But the president declined to say if the US government had evidence China had helped Mr Putin avoid the sanctions placed on Russia, or even sold American equipment to Moscow.
“I think that China understands that its economic future’s much more closely tied to the west than it is to Russia,” said Mr Biden.
“And so I am hopeful that he does not get engaged,” said Mr Biden in reference to Chinese president Xi Jinping.
Nato has said it will provide Ukraine with equipment to protect against chemical, biological, radiological as well as nuclear weapons.
Mr Biden said before he left on his European trip that there was a “real threat” of Russia using chemical weapons.
The Kremlin has previously ordered the use of chemical weapons in Syria, and has poisoned critics such as Alexei Navalny and former Russian spy Sergei Skripal and his daughter.
