Russia and Belarus are due to start 10 days of joint military drills as concerns rise over the build-up of Russian forces on Ukraine’s borders.
Nato says the joint drills mark Russia’s biggest deployment to ex-Soviet Belarus, since the Cold War.
The US called the drills an “escalatory” action in the tensions over Ukraine.
Russia has repeatedly denied any plans to invade Ukraine despite amassing more than 100,000 troops at the border.
But some Western countries including the US have warned that a Russian attack could come at any time.
Diplomatic talks aimed at resolving the conflict are expected across Europe on Thursday.
In 2014 Russia annexed Ukraine’s southern Crimea peninsula. Since then there has been a long-running conflict in eastern Ukraine, where Russian-backed separatists control swathes of territory and at least 14,000 people have been killed.
Some 30,000 Russian troops are expected to take part in the drills with Belarus.
Belarus leader Alexander Lukashenko is a firm ally of Russian President Vladimir Putin. The Kremlin backed Mr Lukashenko when huge protests erupted in Belarus in 2020, while most Western countries imposed sanctions and refused to recognise election results widely believed to have been rigged in the long-time leader’s favour.
A Kremlin spokesman described the joint drills as serious, saying Russia and Belarus were being “confronted with unprecedented threats”.
Russia’s EU ambassador Vladimir Chizhov, however, told the BBC that his country still believed diplomacy could help de-escalate the crisis over Ukraine. He said Russian troops currently stationed in Belarus would return to their permanent bases after the exercises.
White House spokesperson Jen Psaki said: “As we look at the preparation for these military exercises, again, we see this as certainly more an escalatory and not a de-escalatory action.”
French President Emmanuel Macron said talks on resolving the conflict would be revived as early as Thursday and include Russia and Ukraine along with France and Germany – known as the Normandy quartet.
Mr Macron told reporters on Wednesday that Mr Putin had assured him Russian forces would not ramp up the crisis, but Russia said it had given no such guarantee.
After two days of intense diplomacy led by Mr Macron, there is some suggestion that a renewed focus on the so-called Minsk agreements – which sought to end the conflict in eastern Ukraine – could be used as a basis to defuse the current crisis.
Ukraine, Russia, France and Germany backed the accords in 2014-2015.
Some diplomats say the agreements could offer a route to de-escalation, with France’s ambassador to the US, Philippe Etienne, tweeting they should be used to “build a viable political solution”.
German Chancellor Olaf Scholz will meet Baltic leaders in Berlin on Thursday.
“The task is that we ensure the security in Europe, and I believe that that will be achieved,” he said at a joint press conference with Danish Prime Minister Mette Frederiksen on Wednesday.
UK Prime Minister Boris Johnson is set to travel to Brussels and Warsaw on Thursday in support of Nato allies.
Mr Johnson’s trip is part of a surge of diplomatic activity, with Foreign Secretary Liz Truss and Defence Secretary Ben Wallace also due to meet their Russian counterparts in Moscow on Thursday.
Ahead of the first visit to Russia by a UK foreign secretary in four years, Ms Truss said she was determined to stand up for freedom and democracy in Ukraine and intended to urge Moscow to pursue a diplomatic solution.
Moscow has repeatedly blamed what it calls “the Anglo-Saxon nations” for escalating tensions around Ukraine.
And it described the UK’s claims that the Kremlin planned to install a pro-Russian puppet government in Ukraine as hysteria.