Russia has sent arms and drone aircraft into eastern Ukraine and is concentrating troops along the tense border, a buildup that is raising concern about a new escalation in the conflict, the
Russia, which is supporting separatists battling the central Ukraine government in Kiev, has built up its air defense systems to the highest levels since August, Marie Harf, a State Department spokeswoman, said in a statement.
Russia now has deployed more troops and military material on its border with Ukraine than at any time since October, when fighting in eastern Ukraine was heavy, she said.
The Russians and separatists are conducting increasingly complex training missions that leave "no doubt" that Russian troops are present in eastern Ukraine, despite Moscow's denials, the statement said.
The training operations involve Russian drone aircraft that are "unmistakable signs of Russia's presence," Harf said.
The Russians and their separatist allies "continue to violate the terms" of a cease-fire agreement signed Feb. 15 in Minsk, Belarus, Harf said.
The cease-fire sought to end months of fighting between the Kiev government and the separatists.
But the cease-fire has been violated repeatedly, and Western officials are worried that Moscow is planning to again escalate the conflict.
Andrew Weiss, a Russia specialist at the non-partisan Carnegie Endowment for International Peace in Washington, said, "There is palpable concern in Washington that a brief lull is only temporary."
Experts worry, he said, that Moscow will want to spark new fighting to undermine the Ukrainian government's efforts to stabilize the battered country.
But there remains deep uncertainty in Washington about the Kremlin's goals, he said.
Tension has been increased in recent days by the Pentagon's decision to send 250 U.S. troops to train Ukrainian forces, a move Russia denounced as a "provocation."