The United States says Russia has deployed more air defense systems into eastern Ukraine and is involved in training exercises of separatist forces in the area in breach of a cease-fire agreement.

U.S. State Department spokeswoman Marie Harf made the accusations on April 22, after Russia’s foreign minister criticized the United States for exploiting the Ukrainian crisis.

“The Russian military has deployed additional air defense systems into eastern Ukraine and moved several of these nearer the front lines,” Harf said in a statement. This is the highest amount of Russian air defense equipment in eastern Ukraine since August [2014]."

Harf also said that the increasingly complex nature of the training exercises of rebels forces "leaves no doubt that Russia is involved" in the training.

The statement said that Russia is also shipping heavy weapons into Ukraine’s east and is building up its forces along the border.

"After maintaining a relatively steady presence along the border, Russia is sending additional units there," it said, adding that the increase gave Russia its largest presence on the border since October 2014.

Harf said Russian and separatist forces maintain a “sizable number of artillery pieces and multiple rocket launchers” within areas prohibited under the European-brokered cease-fire agreement signed in Minsk in February.

The fragile truce, which took effect on February 15, has reduced the fighting between government forces and pro-Russian rebels in eastern Ukraine that has killed more than 6,000 people in the past year.

But the OSCE's Special Monitoring Mission to Ukraine said last week it had seen a "massive" increase in the number of cease-fire violations in recent days.

Kyiv, NATO, the United States, and several western countries have long accused Russia of sending troops and weapons to support the separatists, a charge Moscow denies.

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 the Minsk deal is fulfilled.

Earlier on April 22, Russian Foreign Minister Sergei Lavrov said Ukraine's unity and neutrality were in Russia's best interest.

In an interview with Moscow-based radio stations Ekho Moskvy, Govorit Moskva, and Sputnik, Lavrov said "It is in our interest not to divide Ukraine. It is in our interests to keep it neutral, primarily in a military-political sense."

Lavrov also accused the United States of using the Ukrainian crisis to reach what he claimed was a "strategic goal" of Washington to "hinder the development of Russia's cooperation with the EU, especially with Germany."

The Russian foreign minister expressed doubts about the effectiveness of a U.S. program to assist Ukrainian military forces with instructors, claiming that such attempts were unsuccessful in Iraq and Afghanistan.

Earlier this month, some 300 U.S. paratroopers arrived in western Ukraine to train with Ukrainian national guard forces. Moscow warned that the move "could seriously destabilize" the situation in Ukraine.

Lavrov also accused the United States of breaching the Non-Proliferation Treaty (NPT) by placing tactical nuclear weapons in Europe.

With reporting by Reuters, Ekho Moskvy, and Interfax