Russia has up to 190,000 troops "in and around" Ukraine, the US ambassador to the Organization for Security and Cooperation in Europe (OSCE) said on Friday.
It came as the US and its allies continued to warn that Russia is on the brink of an invasion.
Michael Carpenter told the OSCE — a body that includes Russia, Ukraine, the US, and more than 50 other nations — that the US has assessed that Russia has between 169,000 and 190,000 personnel there.
He said that number had increased recently, from around 100,000 on January 30.
Carpenter did not explain what he meant by claiming that Russian troops could be "in" Ukraine.
The US and international community recognizes regions that Russia has claimed as still being part of Ukraine, and Russia has long supported the estimated 30,000 separatists fighters on the ground.
The UK Defence Secretary, Ben Wallace, recently said that some "not conventional" advance forces were already in Ukraine.
Russia also claims Crimea as its own territory after its annexation from Ukraine in 2014, and has its own forces there. Russia also offers support to the breakaway Luhansk and Donetsk regions in Ukraine's east.
Russia had said on Tuesday that it was withdrawing troops from Ukraine's border. But Ukraine said it had not seen any evidence, and both the US and NATO said that Russia appeared to only be adding troops.
The figure of 190,000 is the highest estimate the US has given so far.
A senior Biden administration official told reporters on Wednesday night that there were around 160,000 troops near Ukraine's border, NBC News reported.
The official that included around 7,000 troops the official said had recently joined.
