Joe Biden has said the use of chemical weapons by Russia in Ukraine “would trigger a response in kind” by Nato.
The US president, who is in Europe for talks with Western leaders, has not spelt out what the response would be.
After being asked at a news conference whether the use of chemical weapons by Vladimir Putin would prompt a military response from Nato, Mr Biden said: “We would respond if he uses it. The nature of the response would depend on the nature of the use”
“You’re asking whether Nato would cross. We’d make that decision at the time,” Mr Biden said.
Allies spent Thursday discussing next steps to counter Russia’s month-old invasion - including how they might respond should Mr Putin deploy chemical or biological weapons.
They met in a trio of emergency summits in Brussels for Nato, G7 and the 27-member European Council.
Boris Johnson has said it would be “catastrophic” if Mr Putin used chemical weapons, while Nato Secretary General Jens Stoltenberg has made it clear it would result in severe consequences.
Mr Stoltenberg said that Nato leaders agreed on Thursday to send equipment to Ukraine to help protect it against a chemical weapons attack.
White House officials said that both the US and Nato have been working on contingency planning should Russia deploy non-conventional weaponry.
Nato has specially trained and equipped forces if there should be such an attack against a member nation’s population, territory or forces. Ukraine is not a member.
Mr Stoltenberg said in an NBC News interview that if Russia deployed chemical weapons, that would make “an unpredictable, dangerous situation even more dangerous and even more unpredictable”. He declined to comment about how the alliance might respond.
A White House official said Mr Biden’s words did not imply any shift in the US position against direct military action in Ukraine.
Mr Biden and Nato allies have stressed that the US and Nato would not put troops on the ground in Ukraine.
Mr Biden is travelling to Poland on Friday where more than two million Ukrainians have fled from the fighting.
The US president is to visit Rzeszow where energy and refugee issues are expected to be at the centre of talks with President Andrzej Duda.
He will get a briefing on humanitarian aid efforts to assist fleeing refugees and he will meet with US troops from the 82nd Airborne Division who have been deployed in recent weeks to bolster Nato’s eastern flank.
Biden and western allies have pledged new sanctions and humanitarian aid in response to Putin's assault on Ukraine
Billions of dollars of military hardware have already been provided to Ukraine. A US official, who requested anonymity to discuss internal deliberations, said western nations were discussing the possibility of providing anti-ship weapons amid concerns that Russia will launch amphibious assaults along the Black Sea coast.
Mr Biden said his top priority at Thursday’s meetings was to make certain that the West stayed on the same page in its response to Russian aggression against Ukraine.
“The single most important thing is for us to stay unified,” he said.
