The Pentagon has denied claims from Moscow that US troops are training Ukrainian forces in the conflict zone in east Ukraine.
Russian Defence Ministry spokesman Major General Igor Konashenkov said that US forces were not just training forces in west Ukraine, "as Ukrainian TV channels show, but directly in the combat zone in the area of Mariupol, Severodonetsk, Artyomovsk and Volnovakha," reports Russian news agency Interfax.
However the Pentagon said the claims were an attempt to detract attention from Russia's role in fomenting unrest in east Ukraine, where pro-Moscow rebels are battling Ukrainian forces.
"This is a ridiculous attempt to shift the focus away from what is actually happening in eastern Ukraine," said Eileen Lainez, a Pentagon spokeswoman.
"Russia continues to supply lethal weapons, training and command and control support for armed separatists in eastern Ukraine, in blatant violation of Moscow's Minsk commitments and Ukraine's sovereignty," she added.
On Monday, the Pentagon announced that 300 US paratroopers from the 173rd Airborne Brigade had begun a six month training programme with volunteers from Ukraine's national guard in Yavoriv, western Ukraine.
Moscow warned that the move could further destabilise the region, where 6,000 have been killed in fighting in the year long conflict.
In a phone call on Wednesday, US Secretary of State John Kerry asked Russian Foreign Minister Sergei Lavrov to pull Russian forces out of east Ukraine, where he alleges Russian forces train rebels, provide weaponry, and operate surface-to-air missile sites.
Violations of a February peace deal between the two sides are frequently reported.