"In the end, it will prove impossible for Putin to hold down a country that will not accept his rule," Johnson told a news conference after a NATO summit in Madrid.

Agreement to any peace deal would be entirely up to Ukraine, he added, while stressing unity against Russia from the NATO alliance and from G7 countries at a leaders' summit in Germany.

"We've seen what Ukraine can do to drive the Russians back," Johnson said.

"We've seen what they did around Kyiv and Kharkiv, now on Snake Island. I think the right thing for us is to keep going on the course that NATO has set out, no matter how difficult."

At the G7 summit, Johnson joked to fellow leaders that they could emulate Putin by posing topless to show their toughness.

The Russian leader responded that seeing Johnson and the other leaders half-naked would be "disgusting".

The prime minister refused to extend the war of words, arguing that Putin should reflect instead "about the consequences of his barbaric actions", including NATO's planned admission of Finland and Sweden.

"The most important conclusion that Vladimir Putin needs to draw from today and the last few days at NATO, and previously in the G7, is we are totally united in condemning what he has done in Ukraine," Johnson said.

Driven by Russia's actions, NATO members were committed to spending at least 2.0 percent of their gross domestic product on defence, he stressed.

Britain "will reach 2.5 percent of GDP on defence by the end of the decade", he added, although a row has erupted in London over whether Johnson's government is indeed sticking to that commitment.