Boris Johnson has threatened “severe consequences” for a chemical attack on Ukraine - but refused to outline details.
Speaking at a press conference in Brussels after a meeting of NATO leaders, the Prime Minister said there was "visceral horror" at the prospect of Vladimir Putin deploying weapons of mass destruction.
But refused to outline details of the threat and blamed "logistics" for not agreeing to Volodymyr Zelensky's request for the West to send tanks.
“If Putin were to engage in anything like that the consequences would be very severe," he said.
“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.”
A Western official warned such an attack would “fundamentally change” the nature of the month-long war.
During three hours of talks at the coalition’s Brussels headquarters, Mr Johnson told counterparts: "We all want Putin to come to his senses, put his tanks into reverse - he's gonna grind on, only has forward gears.
“He's already crossed the red line into barbarism.
"The heroism of the Ukrainians has fundamentally changed the geopolitics of Europe.
"People will ask, ‘did we do enough’? I don't think we can deny them in their moment of agony. We have the means and they have need."
But the PM blamed “logistics” for being unable to help Mr Zelensky with tanks and fighter planes.
“Logistically at the moment it looks very difficult, both with armour and with jets,” he said.
“We are very conscious of what he’s asking for. At the moment we’re looking at the equipment that we think is more immediately valuable.”
NATO countries are poised to send Hazmat suits to Ukrainian civilians amid fears Russia will unleash chemical weapons.
Coalition leaders agreed to activate the alliance’s chemical, biological, radiological and nuclear defence elements, said Secretary-General Jens Soltenberg.
He told a press conference: “Allies agreed to supply equipment to help Ukraine protect against chemical, biological, radiological and nuclear threats.
“This could include detection equipment, protection and medical support, as well as training for decontamination and crisis management.”
Mr Stoltenberg said: "We are concerned, partly because we see the rhetoric and we see that Russia is trying to create some kind of pretext - accusing Ukraine, the United States, NATO allies - for preparing to use chemical and biological weapons."
Any use of chemical weapons "will totally change the nature of the conflict, it will be a blatant violation of international law and it will have widespread consequences", he warned.
Kits are set to be sent to Ukrainians to protect themselves. But there will not be enough for the entire population.
Western officials said on Thursday: “There is protective equipment which we can supply to Ukrainians such as wash-up kits and protection kits and so on.
“There’s a question as to when and how to provide those and that’s something we need to work through because we don’t have 44 million of these kits and clearly we don’t know where or when or how they would use such weapons.
“There is a lot of work going on in the detail to prepare.”
But the official admitted it was “highly unlikely that NATO would go directly into conflict with Russia ” in response to a chemical atrocity.
Mr Stoltenberg cited the 2018 novichok nerve agent attack in Salisbury, when Russia agents poisoned a former KGB spy in a botched assassination attempt.
NATO rebuffed pleas from Ukrainian President Volodymyr Zelensky to supply “one percent” of the alliance’s tanks and warplanes to help fight invaders.
In a passionate video link plea to NATO leaders, he fumed: “You have thousands of fighter jets but we haven't been given any yet.
“We asked for tanks so that we can unblock our cities that are now dying - Mariupol, Berdyansk, Melitopol, others - cities where Russia is keeping hundreds of thousands of people hostage and artificially creating famine; no water, no food, nothing there.
“You have at least 20,000 tanks; Ukraine asked for a percent - one percent of all your tanks to be given or sold to us - but we do not have a clear answer yet.
“The worst thing during the war is not having clear answers to requests for help.”
He added: "We just want to save our people, we want to survive.”
However, he did not repeat his request for NATO to enforce a no-fly zone above Ukraine.
Mr Stoltenberg signalled tanks and fighter planes would not be deployed for fear of triggering a Third World War.
He said: “We have a responsibility to prevent this conflict from becoming a full-fledged conflict in Europe involving not only Ukraine and Russia but NATO allies and Russia.
“That will be more dangerous and more devastating.”
Meanwhile, the Kremlin branded Mr Johnson “the most active anti-Russian leader”.
Spokesman Dmitry Peskov claimed: "We see him as the most active participant in the race to be anti-Russian."
The PM hit back: “I don’t think there’s a single person around the table in NATO or the G7 who is against Russians or the Russian people, least of all me.
“I’m not remotely anti-Russian but what we all agree is that what Vladimir Putin is doing, the way he is leading Russia at the moment, is utterly catastrophic.”
