Russia has assembled 70 percent of the military force it would need to launch an invasion of Ukraine, according to US officials.
There has been a build-up of Russian troops on the border over recent weeks, fuelling Western fears that an invasion could be imminent.
The officials - who agreed to divulge details of the latest government assessment on condition of anonymity - said Russia has almost three quarters of the forces it likely plans to have in place by mid-month.
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Russia reportedly has around 100,000 troops stationed near the border, but has denied that it is planning an invasion.
The US officials said that Russia could capture the Ukrainian capital of Kyiv within 48 hours, should it launch an invasion.
They also warned that an invasion could result in up to 50,000 civilians being killed or wounded within weeks, while 5,000 to 25,000 Ukrainian troops could be killed or wounded.
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A further five million people could also become refugees.
Speaking yesterday (Saturday 5 February), the officials said the number of battalion tactical groups in the border region has risen to 83 from 60 over the past two weeks.
As for the timing of a potential invasion, the officials said the ground is expected to reach peek freeze on 15 February, which would enable heavier machinery to be mobilised off-road. These conditions will last until the end of March.
Reuters said the officials did not provide supporting evidence for the assessments.
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The officials also stressed that a diplomatic solution appears to remain a possibility. They said the US anticipates Russia may opt for an alternative to a full-scale invasion - such as a limited incursion - but believe President Vladimir Putin has not yet decided and is putting together a force which leaves all options on the table.
US president Joe Biden has ordered 2,000 US-based troops to Poland and Germany and shifted 1,000 more from Germany to Romania in a show of the US commitment to Nato's eastern flank.
Meanwhile, UK prime minister Boris Johnson held a call with French president Emmanuel Macron yesterday, with the leaders agreeing to work to fight Russian aggression 'wherever and however it might occur'.
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"They agreed that finding a diplomatic solution to the current tensions must remain the overriding priority," a No 10 spokesman said.
"The Prime Minister and President Macron stressed that Nato must be united in the face of Russian aggression. They agreed to continue to work together to develop a package of sanctions which would come into force immediately should Russia further invade Ukraine.
"The leaders discussed their work to strengthen Nato's Eastern flank, ensuring that allies are fully defended against malicious Russian activity, wherever and however it might occur."
Topics: World News, Russia, US News