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Russia is building up its air defenses inside of eastern Ukraine and bolstering forces along the border in violation of a recent truce, the State Department said on Wednesday.

"The Russian military has deployed additional air defense systems into eastern Ukraine and moved several of these nearer the front lines," State Department spokeswoman Marie Harf said in a statement.

"Russia is also building up its forces along its border with Ukraine," the statement added. "After maintaining a relatively steady presence along the border, Russia is sending additional units there."

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According to the State Department, the additional forces will give Russia its largest presence on the border since October 2014.

More than 6,000 people have been killed since the fighting in eastern Ukraine erupted more than a year ago. In recent weeks fighting has intensified despite a ceasefire signed in February.

Moscow was also shipping heavy weapons into eastern Ukraine and Russian-separatist forces were conducting increasingly complex training in the restive eastern part of the country, including with drones, in an "unmistakable sign of Russia's presence," Harf added.

Russia has consistently denied sending soldiers and weapons to the pro-Russian rebels. The West has slapped sanctions against Russia over its role in Ukraine, and said the measures will be lifted once Russia fulfills the ceasefire signed in Minsk.

IN-DEPTH

— F. Brinley Bruton

Reuters contributed to this report.