Boris Johnson has said he will warn President Vladimir Putin to "step back from the brink" when the pair hold crisis talks this week, with the UK and western allies seeking to prevent an invasion of Ukraine.
The prime minister - who is set to visit the region this week - said any invasion of Ukraine will be "an absolute disaster for the world but above all it would be a disaster for Russia".
His warning comes with around 100,000 Russian troops amassed on the border with Ukraine, a move by Moscow that has caused serious concern around the world that an invasion is imminent.
Britain has been assisting Ukraine with bolstering its defences since Russia invaded Ukraine in 2014 and annexed the Crimean Peninsula, with UK soldiers providing training and the government supplying weaponry.
Foreign Secretary Liz Truss said on Sunday that the prospect of UK troops fighting alongside Ukrainians against Russia is "very unlikely".
But Mr Johnson said he is confident the Ukrainian people "will fight", adding "any Russian invasion will be bitterly and bloodily resisted".
Ukrainian civilians living near the border with Russia have been receiving military training so they can protect themselves from any incursion.
Hungarian leader Viktor Orban, seen as one of Mr Putin's closest partners in Nato and the European Union, is expected to travel to Moscow on Tuesday for talks which have led to concerns about the alliance being undermined.
Defence Secretary Ben Wallace, who was in Budapest for talks with his counterparts, insisted he supports Mr Orban's visit, telling a press conference: "I think it's very important that President Putin hears from a country such as Hungary that they will face direct economic consequences of any instability in the east.
"I don't know what the prime minister of Hungary will actually say to Putin but I think it is clear that we all are in agreement that we don't want instability, we don't want war in the east, we don't want casualties, we don't want migrant flows, we don't want high fuel prices and food prices which would inevitably follow from any actions."
Foreign Secretary Truss is expected to visit Ukraine this week and will fly to Moscow for talks within the next fortnight.
She has promised new legislation announced to widen current sanctions so "there will be nowhere to hide for Putin's oligarchs" if Russia invades Ukraine.
The UK is expected to bolster military presence in eastern Europe as part of Nato's efforts to secure the region.
But British and Nato combat forces are not expected to be deployed in Ukraine, which is not a member of the alliance.
Around 100 British personnel are involved in a training mission in the country.
Mr Johnson said: "We have been there since 2014, training Ukrainian troops under Operation Orbital - about 21,000 we have trained.
"We give lethal but defensive weaponry to Ukraine."