Boris Johnson has warned of “catastrophic” consequences for Russia should Vladimir Putin use chemical weapons in Ukraine, though stopped short of saying that would include a military escalation.

Speaking after two extraordinary meetings of Nato and the G7, the prime minister also admitted allies were constrained in delivering tanks and jets to Ukraine, despite a direct plea by the Ukrainian president, Volodymyr Zelenskiy.

Johnson warned the UK is unlikely to supply jets and tanks to Ukraine, amid calls from Zelenskiy for the new heavy weaponry.

In a speech at the summit by video link, Zelenskiy did not repeat his call for a no-fly zone but said Ukraine needed jets and tanks from Nato to aid its besieged cities.

“You have at least 20,000 tanks! Ukraine asked for a per cent, 1% of all your tanks to be given or sold to us. But we do not have a clear answer yet,” he said. “The worst thing during the war is not having clear answers to requests for help.”

Johnson said that the UK was “looking at what we could do to help” with tanks but suggested it was impossible. “Logistically, the moment it looks very difficult for both the armour and with jets. We’re very conscious of what he’s asking for.

“At the moment, we’re looking at the equipment that we think is more immediately valuable … They need protection from that absolutely merciless onslaught from artillery.”

The prime minister said at his meeting with the G7, his Japanese counterpart had spoken movingly about the use of nuclear and chemical weapons. “There is a visceral horror of the use of these weapons of mass destruction,” he said.

Johnson said in an interview as he departed the summit that he feared Putin intended to level Ukrainian cities as he did in Chechnya. “Putin doesn’t take peace talks seriously – he wants to double down and Groznyfy Ukraine’s great cities,” he told BBC Newsnight. “I think that’s a tragic mistake, but that’s what he seem to be doing at the moment.”

Johnson would not commit to future Nato membership for Ukraine, something Zelenskiy has said he could be prepared to forgo as part of peace negotiations. Johnson said allies would instead be “strengthening the quills of the Ukrainian porcupine as to make it in future indigestible to the Russian invaders”.

Speaking earlier, Johnson said the US president, Joe Biden, was right to warn about the risk of a staged false flag attack. “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,” he told a press conference in Brussels.

“When the Russians start doing stuff about ‘there are factories in Ukraine producing American biological weapons’ you know that is a prelude to a false flag operation.”

Behind the scenes, officials say there is virtually zero prospect of a military intervention. “I think very few people think that Nato entering into conflict with Russia will stop the war – it will escalate the war,” one western 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.”

Nato leaders agreed Ukraine required supplies of protective equipment against chemical and biological weapons, which Biden has expressed particular concern could be in the offing.

But leaders agreed it was clear that any chemical attack spilling across the border into a Nato country would not be considered a direct attack on the alliance.

Both the US and the UK are expected to begin supplies to Ukrainian civilians, though officials acknowledged it was difficult to know where to target supplies. “There is protective equipment which we can supply to Ukrainians such as wash-up kits and protection kits and so on,” one official aid.

“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.”

Overnight, the prime minister said the UK would double the number of British missiles sent to Ukraine with an additional 6,000, and send an extra £25m to Ukraine’s military. During their three-hour meeting, the 30 heads of government discussed the limits of upgrading weapons supply to Ukraine, without provoking Russia further.

Johnson also denied claims from Kremlin spokesperson Dmitry Peskov that he was “most active anti-Russian leader” – though privately No 10 has been delighted at the description.

“I think it’s very, very important for everybody to understand that there’s not a single person around the table in Nato, or the G7 who is against Russians or the Russian people. Least of all me, the only prime minister in UK history to be called Boris. I’m not remotely anti-Russian. But I think what we all agree, is that what Vladimir Putin is doing, the way he’s leading Russia at the moment is actually catastrophic.”