He said the UK would not stand by while Vladimir Putin "vents his fury on Ukraine" and would work to ramp up defensive weaponry for the country.

Speaking in Brussels, he warned that, if the Russian president used chemical weapons, the consequences would be "catastrophic for him".

Earlier, the UK announced sanctions on 65 more Russian groups and individuals.

Mr Johnson later told BBC Newsnight that Russia did not want peace, and instead wanted to intensify its attack on Ukraine. 

Leaders from Nato, the EU and the G7 have been holding emergency meetings in Brussels to discuss the conflict.

Speaking at a news conference following the Nato summit, Mr Johnson defended the level of the UK's support for Ukraine, saying the government planned to send 6,000 more missiles to the country as well as an extra £25m in aid to help Ukraine pay the salaries of its armed forces.

The PM said kit would be provided to Ukraine to defend against "its bullying neighbour".

Mr Johnson also promised a new deployment of UK troops to Bulgaria, on top of doubling troops both in Poland and in Estonia.

It follows Nato's earlier announcement that new battle groups would be created in Bulgaria, Hungary, Romania, and Slovakia.

"The message Putin can take is: Ukraine is not alone. We stand with the people of Kyiv, Mariupol, Lviv and Donetsk," Mr Johnson said.

"As President Zelensky himself has said, the people of Ukraine must prevail and Putin must fail - and he will."

Mr Johnson added that Western nations were looking to "steadily ratchet up" the amount of military gear they are sending Ukraine, but that it was proving "difficult" to meet the country's request for warplanes and tanks.

In a virtual appearance at the summit earlier, Ukrainian President Volodymyr Zelensky's asked Nato for "1% of all your planes, 1% of all your tanks".

Mr Johnson said: "What President Zelensky wants is to try to relieve Mariupol and to help the thousands of Ukrainian fighters in the city. To that end he does need armour, as he sees it."

"We are looking at what we can do to help. But logistically it looks very difficult both with armour and with jets." 

Mr Johnson added that no Western power was looking to put "boots on the ground" or impose a no-fly zone over Ukraine.

He acknowledged that the Ukrainian president wanted more from Nato, saying allies felt "agony" about their "inability to do more given the constraints we face".

In a statement reported by Russia's Ria news agency, Russia's foreign ministry said Nato's decision to continue supporting Ukraine showed the military alliance wanted the conflict to continue.