Joe Biden raised the stakes with Russia over its unprovoked invasion of Ukraine by warning that Nato would be forced to respond if the Kremlin resorted to using chemical weapons.

Speaking in the aftermath of emergency Nato and G7 summits in Brussels, the US president said that any retaliation would be proportional, though he would not confirm that he would insist on military action.

“We would respond,” Biden said, in response to a question about what Nato would do. “We would respond if he uses it. The nature of the response would depend on the nature of the use.”

The US wanted to see Russia thrown out of the G20 club of leading economic nations, although Biden complained that wasn’t immediately possible because “Indonesia and others do not agree”.

Biden and other western leaders have become fearful that a struggling Russia may try to end Ukraine’s stubborn resistance in the war that has already lasted a month and claimed thousands of lives, with a chemical – or biological or nuclear – attack.

Joe Biden said he supports removing Russia from the G20 organization of major economies, following the invasion of Ukraine.

The topic was raised during his meetings with world leaders in Brussels on Thursday, Reuters reported.

“My answer is yes, depends on the G20,” Biden said, when asked in a press conference if Russia should be removed from the group.

Biden said if the G20 could not agree on removing Russia, then Ukraine should be allowed to attend the meetings.

The president said the key thing was for Nato countries to remain allied against Vladimir Putin.

“The single most important thing is for us to stay unified and the world to continue to focus on what a brute this guy is and all the innocent people’s lives that will be lost and ruined and what’s going on,” Biden said.

The UK and allies will “ramp up” lethal aid to Ukraine, Boris Johnson said following a meeting of G7 leaders. The UK will send an additional 6,000 missiles and provide £25m in funding for Ukraine’s armed forces, he said, with kit provided “in the quantity and with the quality” needed by Ukraine to defend itself against “its bullying neighbour”.