Russia's defense ministry announced Monday that 100 fighter jets, helicopters and bombers will conduct military exercises near the Ukrainian border this week, according to the BBC.
The move comes days after the U.S. and the European Union slapped new sanctions on Russia, and amid calls for Moscow to stop supplying eastern Ukrainian separatists with heavy weapons, tanks and other assistance.
"We're certainly aware of these reports, and if true, this would be highly provocative and would serve only to escalate tensions instead of deescalate tensions," Pentagon spokesman Army Col. Steve Warren said Monday.
"It's another example of the Russians conducting themselves in a manner exactly opposite of what they said they would do," he added.
Ukrainian forces have gained ground in recent days against pro-Russia militants, raising fears Moscow would again intervene.
The Pentagon said Friday that more than 10,000 Russian troops are massed at Ukraine's southeastern border, despite U.S. and international calls for them to return to their bases.
Lawmakers are pressing the Obama administration to fulfill requests by the Ukrainian military for lethal assistance, which could include anti-aircraft weapons for use in the event of a Russian incursion. U.S. officials, though, say no decisions have been made.
A Russian spokesman said the military exercise was the first in a series intended to improve air force operations and would include Sukhoi Su-27 and MiG-31 fighter jets, Mi-24 and Mi-28 helicopters, and Russia's newest bomber, the Sukhoi Su-34.
This story was updated at 12:20 p.m.