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.

This week it emerged that the White House had set up a special “Tiger Team” to work out how the US would respond if such an attack should take place. But on Thursday, Biden also said: “It would trigger a response in kind” – meaning it would reflect what had taken place.

Concern about a possible Russian chemical or biological attack dominated the Nato summit on Thursday. Speaking a few minutes before Biden, UK prime minister Boris Johnson warned of disastrous consequences for Russia should Putin use chemical weapons in Ukraine.

“You have to have a bit of ambiguity about your response, but I think it would be catastrophic for him if he were to do that – and I think that he understands that,” Johnson told a press conference.

All 30 Nato leaders warned Russia against using chemical or biological weapons in Ukraine, arguing in a final summit statement that any such use “would be unacceptable and result in severe consequences”. However, again they did not spell out what the response would be.

One western official said on Thursday they thought it would be “highly unlikely” that there would be a direct military response even if Russia used chemical weapons in Ukraine, although any final decision would be driven by the US.

That would risk a severe escalation of the conflict, the official said. “We are remaining deliberately ambiguous. We don’t want to say exactly how we will respond because Putin will then price that in to his calculations. We want him to remain uncertain … but we are clear there will be severe consequences,” they added.

Masks, protective suits, detection and other equipment would also be supplied by Nato members to Ukraine, Nato’s secretary general Jens Stoltenberg confirmed, reflecting western fears that Russia is trying to create “some kind of pretext”.

The alarm grew after Russia began to spread unproven claims this month that Ukraine had been developing a biological weapons programme with the help of the US. This week Biden said such claims were a “clear sign” that Putin was contemplating a biological or chemical assault.

The development, production and stockpiling and use of chemical weapons is banned by an international treaty signed by 193 countries, including Russia and the US.

Chemical weapons were used on at least 17 occasions during Syria’s civil war, according to the Organisation for the Prohibition of Chemical Weapons (OPCW), which has accused the Russian-backed regime of Bashar al-Assad of being behind several high-profile attacks.

Nato leaders are also willing to send more powerful weapons to Ukraine after a plea at the Brussels summit from the country’s president, Volodymyr Zelenskiy, for tanks, rockets and air-defence systems.

On Thursday, during a three-hour meeting, the 30 heads of government discussed how they could upgrade and increase their weapons supply – but without provoking Russia to the point where the conflict would lead to a wider war in Europe.

Stoltenberg said: “I will not go into the details of the exact type of systems we are deploying … What I can say is that allies do what they can to support Ukraine with weapons.”

At the summit, Biden, Johnson and other Nato leaders heard a characteristically passionate call from Zelenskiy for “military assistance – without restrictions”. He asked Nato members to supply more weapons because “Russia uses without restrictions its entire arsenal against us”.

Zelenskiy specifically asked for anti-missile systems, MLRS rocket artillery, anti-ship weapons, air defence equipment – plus tanks and fighter jets. “You can give us 1% of all your planes, 1% of your tanks,” he said.

Nato leaders discussed what further weapons could be supplied, given that Ukraine had held out for far longer than expected, and which countries were best placed to supply what arms, western officials said.

“There was a discussion both about more [military equipment] in terms of more numbers, and more in terms of different equipment,” one official said.

But there are limits; officials indicated they thought it unlikely any Nato country would want to provide tanks, for fear of Russian retaliation. A previous proposal by Poland to transfer 28 MiG-29 jets via the US was rejected by the White House, for similar reasons.

“We have a responsibility to prevent this conflict from becoming a full-fledged war in Europe, involving not only Ukraine and Russia, but Nato allies and Russia,” Stoltenberg said in a post-summit press conference.

Nato members have largely said they will only provide “defensive equipment”, but Russian military errors and Ukraine’s determination to fight back have left the aggressors struggling to make progress without resorting to heavy bombardment that has left the cities of Mariupol and Kharkiv in ruins.

Johnson said the UK would double the number of British missiles sent to Ukraine with an additional 6,000 He also pledged an extra £25m to Ukraine’s military to help it better fight Russia.