Britain and Ukraine warned on Tuesday that any Russian incursion would be a massive mistake, pledging to work together to strengthen Ukraine's defence capability.
The two nations issued a joint statement issued following a meeting between British Prime Minister Boris Johnson and Ukrainian President Volodymyr Zelenskiy.
"The leaders warned that any further Russian incursion in Ukraine would be a massive strategic mistake and have a stark humanitarian cost," the statement said.
Russian President Vladimir Putin is holding a gun to Ukraine's head in an effort to force a change to the architecture of European security, British Prime Minister Boris Johnson said on Tuesday, stressing he was not exaggerating Russia's threat.
"This is a clear and present danger. We see large numbers of troops massing, we see preparations for all kinds of operations that are consistent with an imminent military campaign," Johnson said, adding he would talk to Putin on Wednesday.
"It's about the whole European security architecture, because be in no doubt about what I think President Putin is trying to achieve here. I think that he is trying, by holding a gun as it were to the head of Ukraine, by intimidating Ukraine, to get us to change the way we look at (European security)."
Johnson also warned on Tuesday that a Russian invasion of Ukraine would lead to military and humanitarian disaster.
Russia, which seized Crimea from Ukraine in 2014 and backs separatists in the east of the country, has amassed tens of thousands of Russian troops near Ukraine, leading to fears in the West that Moscow could invade.
"A further Russian invasion of Ukraine would be a political disaster, a humanitarian disaster, and in my view it would also be for Russia and the world a military disaster," Johnson said at a press conference on a visit to Kyiv, urging Russia to pull back troops and choose diplomacy.
"It is vital that Russia steps back and chooses a path of diplomacy, and I believe that is still possible," Johnson said.
British sanctions will be automatically imposed on Russia the moment any incursion into Ukraine takes place, he added.
"It's vital that in Moscow, they understand that there will be automaticity in the way that we apply these sanctions, so that the minute there is a further incursion into sovereign Ukrainian territory then those sanctions will apply," Johnson told a news conference in Kyiv.
"We're bringing forward the new legislation (that) will enable us to pinpoint ... strategic commercial interests of Russia in a very direct way, as well as individual Russian commercial interests."
Russia has denied any plans to invade.
Johnson, speaking alongside Ukrainian President Volodymyr Zelenskiy, said Ukraine would resist any invasion.
"I think that perhaps the single most useful thing we can all do is get over to the Russian public, to citizens in Russia, thinking about this possibility, the reality that the Ukrainian army will fight," Johnson said.
"There are 200,000 men and women under arms in Ukraine, they will put up a very, very fierce and bloody resistance. I think that parents, mothers in Russia should reflect on that fact and I hope very much that President Putin steps back from the path of conflict and that we engage in dialogue."
