BERLIN: German Chancellor Olaf Scholz and British Prime Minister Boris Johnson warned on Friday that Russia would pay a "considerable and serious price" if it invades Ukraine.

In a phone call on Thursday evening, the two leaders agreed that "further military aggression by Russia against Ukraine must be averted", the German chancellery said in a statement.

Downing Street said Scholz and Johnson had "shared their deep concern at the ongoing destabilising action by Russia in Ukraine, and said any invasion into Ukraine would be a severe strategic mistake".

"The prime minister stressed the importance of NATO allies working together on a coordinated response," it said in a statement.

Russia announces huge naval drills as tensions with West soar

Fears are mounting that a major conflict could break out in Europe as tens of thousands of Russian troops mass on Ukraine's border, along with an arsenal of tanks, fighting vehicles, artillery and missiles.

In a bid to defuse the worst tensions between Russia and the West in decades, US Secretary of State Antony Blinken is on a whirlwind diplomatic tour that took him to Berlin on Thursday before a meeting with Russian Foreign Minister Sergei Lavrov in Geneva on Friday.

The United States and its allies have warned Moscow of grave consequences if "any" of the troops massed on the border were to cross the border into Ukraine.