A state department spokeswoman, Marie Harf, said in a statement on Wednesday: “This is the highest amount of Russian air defence equipment in eastern Ukraine since August.”
She said the increasingly complex nature of the training exercises “leaves no doubt that Russia is involved”, adding: “The training has also incorporated Russian UAVs [unmanned aerial vehicles], an unmistakable sign of Russian presence.”
Fighting has picked up in eastern Ukraine in recent weeks despite the ceasefire agreed in February, with each side accusing the other of violence. But Sergei Lavrov, Russia’s foreign minister, said on Monday he believed a surge in ceasefire violations had passed.
Harf, however, 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 on Moscow over its role in the Ukraine crisis, warning that the measures will be lifted only once Russia fulfils the ceasefire signed in Minsk.