“We have continued to see even over the last 24 hours additional capabilities flow from elsewhere in Russia to that border with Ukraine and Belarus,” Pentagon spokesman John Kirby said.
Meanwhile, 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.
In a new round of diplomacy, Britain’s foreign minister sparred publicly with Russia’s top diplomat at talks in Moscow, Johnson 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.
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