UK Foreign Secretary Liz Truss will tell Russia that diplomacy is the only way forward in Ukraine as Britain takes its turn leading European efforts to defuse the crisis.
Ms Truss will use a two-day visit to Moscow to tell Russian Foreign Minister Sergey Lavrov that the Kremlin should be "in no doubt about the strength of her response" if it shuns diplomacy and invades Ukraine.
Her office said she would tell Russia that its posture was damaging its interests and standing, and that any invasion would bring “massive consequences for all involved”.
Downing Street announced on Wednesday that 1,000 British troops were being put on standby in case of a humanitarian crisis in Eastern Europe if the Russian military build-up led to war.
"When Nato was founded, allies made an historic undertaking to safeguard the freedom of every member state," British Prime Minister Boris Johnson said.
"The UK remains unwavering in our commitment to European security. What we need to see is real diplomacy, not coercive diplomacy."
On Monday, French President Emmanuel Macron declared he had made progress in talks with Russia.
But Moscow said negotiations with the UK would be brief if Britain did not ease up on talk of imposing sanctions, including on wealthy Russians who have long used London as a base.
"If they're coming to Russia to threaten us again with sanctions then it is fairly pointless. We read everything, see everything, know and hear," said Andrey Kelin, Russia's ambassador to London.
The Foreign Office said Ms Truss would tell Russia that "the only way forward is for Russia to cease its aggression and engage in meaningful talks".
“Russia has a choice here. We strongly encourage them to engage, de-escalate and choose the path of diplomacy,” Ms Truss said.
“Russia should be in no doubt about the strength of our response. We have said many times that any further invasion would incur severe costs, including through a co-ordinated package of sanctions.”
Ms Truss had been due to accompany Mr Johnson to Ukraine last week but was forced to stay at home after testing positive for Covid-19.
Focus on Minsk
She will use the trip to tell Russia to abide by existing international agreements such as the Minsk protocols, which envisage a peace process in war-torn eastern Ukraine.
Russia and Ukraine blame each other for failing to abide by those accords, which foresee a decentralised Ukraine as a compromise between Kiev and pro-Kremlin separatists.
The US embassy in Kiev on Tuesday accused Russia of misrepresenting that pact by insisting on autonomy, rather than a limited special status, for a separatist-controlled region.
But Mr Macron, who visited Moscow and Kiev this week, said leaders in both capitals had indicated they were willing to enact the Minsk agreements.
He said presidential advisers from France, Germany, Russia and Ukraine will meet in Berlin on Thursday to discuss the next steps.
In the US, President Biden's nominee for top military commander in the Middle East warned that a Russian invasion could create broader instability in the region, including in Syria.
Mr Macron claimed he had extracted a promise from Russia not to escalate matters further, although the Kremlin played down this remark.
Ukraine said on Wednesday that the European diplomatic push was bearing fruit but that tension remained high, with 100,000 Russian troops massed on their border and thousands more in Belarus.
"The situation remains tense but under control. Diplomacy is continuing to lower tensions," Foreign Minister Dmytro Kuleba said.
France held three-way talks with Germany and Poland, which ended in a declaration late on Wednesday that the so-called Weimar Triangle countries were open for further talks with Russia.
While refusing to export weapons to Ukraine, Germany has been keen to emphasise its military support to Nato members, with an air force detachment arriving in Romania on Wednesday as part of an air-policing mission.
Olaf Scholz, Germany's Chancellor, was expected to meet European Council president Charles Michel in Berlin later on Wednesday.
Mr Scholz is also expected in Moscow next week in another round of East-West talks with Russian President Vladimir Putin.
