Britain said on Thursday the West could face the“most dangerous moment” in its standoff with Moscow in the next few days, as Russia held military exercises in Belarus and the Black Sea following its troop build-up near Ukraine.

Tensions remained high, with Ukraine also staging war games, 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 diplomacy, Britain’s foreign minister sparred publicly with Russia’s top diplomat at talks in Moscow, Boris visited Nato headquarters in Brussels and officials from Russia, Ukraine, Germany and France were due to meet in Berlin to discuss the conflict in eastern Ukraine.

Russia, which has more than 100,000 troops near Ukraine’s borders, denies western accusations that 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, Boris told a news conference with Nato secretary-general Jens Stoltenberg in Brussels. “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, and we’ve got to get it right. And I think that the combination of sanctions and military resolve, plus diplomacy is what is in order.”

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 made navigation in the Black Sea and Sea of Azov “virtually impossible”.

Ukraine’s defence minister Oleksii Reznikov later urged the international community to take tough punitive measures on Russia, including port restrictions on its ships, to retaliate.

“Now we expect unified reaction also: when RUS (Russian) ships can’t enter world’s ports, they’ll understand the price of their impudence,” he wrote on Twitter.