WASHINGTON, April 22 – The United States accused Russia Wednesday of building up its forces along the border with Ukraine, boosting air defense systems in the country and training Ukrainian rebels in the east, AFP reported.

"Combined Russian-separatist forces continue to violate the terms of the 'Minsk 2' agreement signed in mid-February," acting State Department spokeswoman Marie Harf said in a statement.

European OSCE observers have reported ongoing heavy arms fire by both the Ukrainian army and pro-Russian separatist militias over recent weeks despite a February ceasefire agreement signed in the Belarus capital of Minsk.

Harf said combined Russian and separatist forces had "a sizable number of artillery pieces and multiple rocket launchers within areas" banned under the ceasefire deal, and were continuing to ship heavy weapons into Ukraine.

The Russian military had also "deployed additional air defense systems into eastern Ukraine and moved several of these nearer the front lines. This is the highest amount of Russian air defense equipment in eastern Ukraine since August."

Moscow was also "conducting increasingly complex training" of the rebels inside eastern Ukraine including with drones "an unmistakable sign of Russia's presence," Harf said.

She also maintained that Russia was building up forces on the border to what would be it "largest presence on the border since October 2014."

Some 300 US paratroopers this week began training about 900 Ukrainian National Guard forces to fight pro-Russian separatists in the east, triggering anger from Moscow.

The United Nations says more than 6,000 people have been killed in the year-long conflict in the past year and more than a million civilians have been displaced.

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. (afp/ez)