British Prime Minister Boris Johnson said Friday there was "still a chance to avoid unnecessary bloodshed" that would follow any Russian invasion of Ukraine.
Downing Street said that Russia continued to strengthen its forces massed on Ukraine's borders, despite denials from Moscow, suggesting that 7,000 troops had bolstered Russian positions, reports AFP.
Washington estimates that up to 190,000 troops, including pro-Moscow separatist fighters in Ukraine's east, could be ready to invade Russia's pro-Western neighbour.
"There is still a chance to avoid unnecessary bloodshed, but it will require an overwhelming display of Western solidarity beyond anything we have seen in recent history," Johnson said in a statement ahead of attending the Munich Security Conference.
"Allies need to speak with one voice to stress to President Putin the high price he will pay for any further Russian invasion of Ukraine. Diplomacy can still prevail."
The annual summit has this year focused on mounting fears that Russia will invade Ukraine.
Ukrainian President Volodymyr Zelensky is due to meet US Vice President Kamala Harris for talks Saturday.
Russia, which typically attends the conference, is not attending this year.