The U.S. has accused Russia of deploying more air defence systems in eastern Ukraine in breach of a ceasefire deal, reports the BBC.
The state department also said Russia was involved in training separatist forces in the area and building up its forces along the border. The Kremlin has not yet responded to the claims. A truce between Ukrainian forces and pro-Russian rebels in east Ukraine was brokered by the West in Minsk in February.
Ukraine accuses Russia of arming the rebels and sending Russian troops over the border - a claim Moscow denies.
State department spokeswoman Marie Harf said in a statement that "combined Russian-separatist forces" were violating the terms of the Minsk deal, keeping artillery and multiple rocket launchers in prohibited areas. "
The Russian military has deployed additional air defence systems into eastern Ukraine and moved several of these nearer the front lines," she said.
More than 6,000 people have been killed since the conflict in eastern Ukraine erupted more than a year ago, reports Reuters, when rebels opposed to the ousting of a Moscow-backed president and the installation of a pro-Western government declared independence from Kiev.
Ukraine and its Western allies have long accused Moscow of sending troops and weapons to support the rebels, a charge Russia has denied.
The United States and Europe have imposed sanctions against Moscow over its role in the Ukraine crisis, warning that the measures will only be lifted once Russia fulfills the ceasefire signed in Minsk.