Britain said the "most dangerous moment" in the West's standoff with Moscow appeared imminent, as Russia held military exercises in Belarus and the Black Sea following the buildup of its forces near Ukraine.
Ukraine also staged war games and the United States urged Americans in the country to leave immediately due to increased threats of Russian military action.
But leaders on all sides signalled they hoped diplomacy could still prevail in what British Prime Minister Boris Johnson called Europe's biggest security crisis for decades.
In a new round of talks, Britain's foreign minister sparred publicly with her Russian counterpart in Moscow, Johnson visited NATO headquarters in Brussels and Germany's leader met his Baltic states counterparts in Berlin, where officials from Russia, Ukraine, Germany and France were also holding discussions.
Russia, which has more than 100,000 troops near Ukraine's borders, denies Western accusations it may be planning to invade its former Soviet neighbour, though it says it could take unspecified "military-technical" action unless demands are met.
"I honestly don't think a decision has yet been taken" by Moscow on whether to attack, Johnson told a news conference with NATO Secretary-General Jens Stoltenberg. "But that doesn't mean that it is impossible that something absolutely disastrous could happen very soon indeed.
"This is probably the most dangerous moment, I would say, in the course of the next few days, in what is the biggest security crisis that Europe has faced for decades."
The way forward was diplomacy, Johnson later told reporters in Poland.
Stoltenberg also said it was a dangerous moment for European security, adding: "The number of Russian forces is going up. The warning time for a possible attack is going down."
In a new point of friction, Ukraine criticised Russian naval exercises that it said were part of a "hybrid war" and had made navigation in the Black Sea and Sea of Azov "virtually impossible".
Nearly nine hours of talks between Ukraine and Russia on Thursday failed to produce a breakthrough on signing a joint document, but both sides agreed to keep talking, the chief of staff to Ukraine's president said after the talks in Berlin.
Russia said the talks with Ukraine, France and Germany on the conflict in eastern Ukraine fell short of any new agreement, and criticised what it called a lack of clarity in the Ukrainian position.
Serbia PM Vucevic formally resigns, triggering deadline for new government
Six migrants dead, 40 missing in shipwreck off Italy's Lampedusa
Netanyahu says Gaza strikes are 'just the beginning'
NASA astronauts 'Butch and Suni' return to Earth
Zelenskyy calls for blocking Moscow's attempts to drag out war
