The next few days could be the most dangerous in Europe's largest security crisis in decades, as Russia conducts war games in Belarus, according to British Prime Minister Boris Johnson, though he believes Moscow has not yet chosen whether to invade Ukraine.

Russia, which has more than 100,000 troops on the Ukrainian border, dismisses Western accusations that it is planning an invasion of its former Soviet neighbour, but warns that if demands are not met, it will take unspecified "military-technical" action.

"I honestly don't think a decision has yet been made" by Moscow on whether to invade, Johnson said alongside NATO Secretary-General Jens Stoltenberg at the NATO military alliance's headquarters in Brussels.

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"That doesn't rule out the possibility of something very disastrous occurring very soon. And, I'm afraid to say, our intelligence remains grim," Johnson said.

"This is perhaps the most dangerous time in the next few days in what is the biggest security issue that Europe has faced in decades, and we have to get it right. And I believe that a combination of sanctions, military force, and diplomacy is required."

Putin, who claims that Moscow's worries about NATO enlargement have been disregarded for three decades, has demanded guarantees that no missiles will be deployed near its borders and that NATO will not be enlarged further.

Britain has taken a firm stance that Russia should not be able to veto countries' decisions to join the Western alliance.

"What we're actually all trying to do, whether you're in NATO or not, is safeguard countries' sovereign right to pick their security alliances," Defence Secretary Ben Wallace said.