Germany warns Russia of 'high cost' of Ukraine aggression

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.

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.

Speaking to journalists in Berlin, Blinken warned that Russia risked reviving Europe's dangerous Cold War era of division as it threatens Ukraine.

"To allow Russia to violate those principles with impunity would drag us all back to a much more dangerous and unstable time, when this continent, and this city, were divided in two... with the threat of all-out war hanging over everyone's heads," he said.

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.

READ ALSO: Germany’s Scholz supports Ukraine amid Russian invasion fears

'Grave consequences'

Appearing alongside Blinken, German Foreign Secretary Annalena Baerbock urged Russia to "take steps towards deescalation". 

"Any further aggressive stance, any further aggression, would have grave consequences," she said. "Nothing less is at stake than the preservation of the European peace order."

Baerbock added that the Western allies would not shy away from taking action, even if that included measures that "could have economic consequences for ourselves".

The recently completed -- but not yet certified -- Nord Stream 2 pipeline, which is due to double Russian gas supplies to Germany, has long been seen as a possible bargaining chip in the crisis.

READ ALSO: German regulator suspends Nord Stream 2 approval process

Although Germany has previously insisted the pipeline was merely a commercial project and should not be halted for political reasons, Chancellor Olaf Scholz recently said "everything" was on the table.

Moscow insists it has no plans to invade Ukraine but has at the same time laid down a series of security demands -- including a ban on Ukraine joining NATO -- in exchange for de-escalation.

In London, British Prime Minister Boris Johnson said any Russian incursion into Ukraine "would be a disaster for the world".