REUTERS - 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 separatists in the area in breach of a European-brokered cease-fire agreement.

"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.

Russia was also building up its forces along the Ukrainian border, she said. "After maintaining a relatively steady presence along the border, Russia is sending additional units there," Harf said, adding that the increase gave Russia its largest presence 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.