Boris Johnson says the UK would be prepared to send extra troops to any new Nato deployments “to protect our allies in Europe” if Russia were to invade Ukraine.
He reiterated, as have many western nations, that the UK would toughen its sanctions regime against Russia if it were to attack Ukraine.
About 100,000 Russian troops are massed on its border with Ukraine, leading to fears in some western capitals that an invasion is imminent.
“We have to beware of doing things … that would constitute a pretext for [Russian President Vladimir] Putin to invade,” Mr Johnson told parliament.
“We have to calculate and calibrate what we do very carefully and I think building a strong package of economic sanctions, continuing to supply defensive weaponry, and all the other things that we're doing – that's the right package.”
On Monday, Nato member states sent reinforcements to eastern Europe to bolster its forces there. Russia has criticised the West’s response as hysteria and insists it has no invasion plans.
“If the worst happens, and the destructive firepower of the Russian army were to engulf Ukraine’s towns and cities I shudder to contemplate the tragedy that would ensue,” Mr Johnson said.
“Ukrainians have every moral and legal right to defend their country and I believe their resistance would be dogged and tenacious, and the bloodshed comparable to the first war in Chechnya or Bosnia, or any other conflict that Europe has endured since 1945. No one would gain from such a catastrophe.”
High-level diplomatic meetings between Russia and the West have so far failed to make headway.
Moscow has demanded several highly controversial security guarantees from Nato, including a ban on Ukraine joining the military alliance. That has been rejected out of hand by Nato, with Mr Johnson saying the proposals “would divide our continent once again".
“We cannot bargain away the vision of a Europe whole and free that emerged in those amazing years from 1989 to 1991,” Mr Johnson said, referring to the fall of the Iron Curtain, the end of Communist rule in eastern Europe and the collapse of the Soviet Union.
“Healing the division of our continent by the Iron Curtain, we will not reopen that divide by agreeing to overturn the European security order because Russia has placed a gun to Ukraine's head.”
Russia has already invaded Ukraine once, annexing the Crimean Peninsula in 2014. It also backed pro-Russian Ukrainian separatists fighting the Kiev government in the Donbass region.
Referring to when he spoke to Mr Putin in December, Mr Johnson said he had emphasised that Nato has no thought of encircling or threatening Russia.
“I said that any attack on his neighbour would be followed by tougher sanctions against Russia, further steps to help Ukraine defend herself and by an increased Nato presence to protect our allies on Nato’s eastern flank.”
