About 300 US paratroopers have come to western Ukraine to train with Ukrainian national guard units, the US Army says.
Kremlin spokesman Dmitry Peskov warned that the move “could seriously destabilise the situation” in Ukraine.
He said the US military presence “is a long way from helping towards a settlement of the conflict”, Russia’s RIA Novosti news agency reported.
The US and its Nato allies accuse Russia of sending soldiers and weapons to the separatists in eastern Ukraine.
Russian President Vladimir Putin again dismissed the Western charges on Thursday, telling millions of Russians that “there are no Russian troops in Ukraine”.
The rebels in the east signed a ceasefire agreement with the Ukrainian government in February, but recently the number of violations has escalated.
There has been further shelling on the outskirts of rebel-held Donetsk and in the village of Shyrokyne, near the southern port city of Mariupol.
Ukraine’s national guard has been involved in the fighting. It includes various volunteer units who are now being integrated with the Ukrainian regular army.
The US Army said the US paratroopers were part of the 173rd Airborne Brigade.
The training will take place at Yavoriv, near Lviv in western Ukraine. The US forces will begin training three battalions of Ukrainian troops over the next six months, the statement said.
The brigade trained with Ukrainian forces in international exercises in Ukraine last September.