Interfax quoted Russian Defense Ministry spokesman Major General Igor Konashenkov as saying U.S. troops were training Ukrainian forces not only in western Ukraine "as Ukrainian TV channels show, but directly in the combat zone in the area of Mariupol, Severodonetsk, Artemivsk and Volnovakha."
The Russian Defense Ministry confirmed the report, but the Pentagon flatly denied it.
"This is a ridiculous attempt to shift the focus away from what is actually happening in eastern Ukraine," said Eileen Lainez, 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.
Lainez said the Pentagon has been clear that it has a few hundred troops training Ukrainian guardsmen in the western part of the country near Poland.
The Pentagon announced on Monday that some 300 U.S. Army paratroopers from the 173rd Airborne Brigade based in Vicenza, Italy, had begun long-planned training for members of the Ukrainian national guard at the International Peacekeeping and Security Center at Yavoriv, near the Polish border.
The initial training involved 300 guardsmen. The U.S. troops are expected to train a total of 900 guardsmen over the course of six months, the Pentagon said. The focus of the training is weapons handling, first aid, casualty evacuation and other small unit activity.
U.S. forces moved ahead with the training despite Russian warnings that it could be destabilizing. A cease-fire is technically in force in eastern Ukraine, but violations are reported daily.
Secretary of State John Kerry discussed the Ukraine situation with Russian Foreign Minister Sergei Lavrov in a phone call on Wednesday, urging Russia to press the separatists to abide by the ceasefire and other steps agreed at Minsk, the State Department said.
Kerry also insisted that Russia abide by all of its Minsk obligations, including "removing Russian forces from eastern Ukraine, where they continue to provide heavy weapons and training to separatists," the department said in a statement.
Washington on Wednesday accused Russia of building up air defense systems inside eastern Ukraine and of involvement in training exercises of pro-Russian rebels in breach of the European-brokered truce.