The United States says Russia continues to reinforce its military presence around the borders of Ukraine, with as many as 7,000 troops recently deployed.
Satellite images show a new pontoon bridge has been built several kilometers from the Ukrainian border in Belarus where Russian troops have been operating.
The BBC's David Willis reports.
White House sources say that in recent days Russia has added as many as 7,000 troops to the nearly 150,000 that are thought to already be in position near the border with Ukraine.
They say some of those troops arrived within the last few hours.
America's claims echo those by the NATO Secretary General, Jens Stoltenberg, who earlier told reporters that as well as increasing its military presence on the border with Ukraine, Russia was sending additional forces.
U.S. officials have warned of the possibility of a so-called false flag operation under which Russia would stage some sort of attack on its own forces and use that as a pretext for invading Ukraine.
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