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Russia has positioned 50% of its ground combat units on the Ukrainian border, the UK Ministry of Defence (MOD) said Thursday.
"It is the largest gathering of Russian troops since the fall of the Soviet Union in 1991," the MOD said in an information bulletin posted to Twitter.
"The scale of this is far beyond that needed for a large scale training exercise."
The MOD did not specify which ground combat units had been stationed at the border. The ministry did not immediately respond to Insider's request for comment.
Russia has amassed as many as 150,000 troops near Ukraine's border since late November, with President Joe Biden warning Thursday that chances of a full scale invasion were "very high."
Russia on Tuesday announced that it was withdrawing some troops from the Ukrainian border following military training.
The US, NATO, UK, and Ukraine have disputed that claim, saying Russia was, on the contrary, increasing its military presence. "They have not moved any of their troops out. They have moved more troops in," Biden said Tuesday.
Russia's defense also announced plans to hold missile test launches on Saturday. The exact location of the launches is unclear.
During a speech at the United Nations Security Council on Thursday, US Secretary of State Antony Blinken said he expects Russia to manufacture "a pretext for its attack" before invading Ukraine. This could be a "violent event" Russia blames on Ukraine or an "outrageous accusation" Russia levels against Kyiv, he said.
The UK has leapt to Ukraine's defense during the current standoff, sending artillery and aid.
"We are deploying troops and assets on land, sea, and air to bolster European defenses in response to the buildup of Russian military forces on the border of Ukraine," UK Defence Secretary Ben Wallace said Thursday. "NATO and our allies have been clear that an invasion of Ukraine will be met with severe consequences. De-escalation and diplomacy remain the only path out of this situation."
Despite the military buildup, Russia has claimed there is still time for a diplomatic solution to the crisis.
The State Department said Thursday that Blinken accepted an invitation to meet Russian Foreign Minister Sergei Lavrov in Europe next week, so long as Russia doesn't invade Ukraine.
Biden is scheduled to speak with NATO leaders on Friday to discuss the crisis.