The United States 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 a European-brokered truce.
"This is the highest amount of Russian air defense equipment in eastern Ukraine since August," State Department spokeswoman Marie Harf said in a statement.
She said the increasingly complex nature of the training exercises "leaves no doubt that Russia is involved."
"The training has also incorporated Russian UAV's (unmanned aerial vehicles), an unmistakable sign of Russian presence," Harf added.
Fighting has picked up in eastern Ukraine in recent weeks despite the cease-fire agreed in February, with each side accusing the other of violence. Russian Foreign Minister Sergei Lavrov said on Monday that he believed a surge in cease-fire violations had however passed.
But Harf said Russia was building up its forces along the Ukrainian border.
"After maintaining a relatively steady presence along the border, Russia is sending additional units there," she said, adding that the increase marked the largest presence of Russian troops on the border since October.
More than 6,000 people have been killed since the conflict in eastern Ukraine erupted more than a year ago, 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.