The State Department warned Moscow that a large Russian military exercise underway near the border with Ukraine could sharpen tensions as separatists battle Ukrainian troops nearby.
U.S. officials are "deeply concerned" by the aviation operation, which has already begun and is expected to continue until Friday, said Jen Psaki, a State Department spokeswoman.
"Exercises of this kind are provocative and only serve to escalate tensions," she told reporters.
Russia announced new military exercises involving bombers and fighter jets on Monday in a show of strength near the border with Ukraine.
The exercise involves dozens of fighters, bombers and attack helicopters.