Russian Defense Minister Sergei Shoigu promised his United States counterpart Chuck Hagel in a telephone call on Thursday that Moscow would not assault eastern Ukraine.
Hagel voiced concern about Russian military movements but Shoigu assured him that "the troops he has arrayed along the border are there to conduct exercises only and they have no intention of crossing the border into Ukraine and that they would take no aggressive action", Pentagon spokesman Rear Admiral John Kirby told reporters.
The US defense secretary also asked how long the military "exercise" would last but Shoigu "didn't have a firm time frame for that", Kirby said.
Shoigu had pledged that Russia would not send troops into Ukraine's east and Hagel's "expectation is that he'll live up to those words", he said.
Washington has watched a Russian build-up on Ukraine's eastern border with growing dismay after Moscow's military intervention in Crimea.
Moscow's promise to Washington came as Ukraine warned it would respond with military action if Russia tries to seize the country's mainly Russian-speaking eastern regions.
NATO Secretary-General Anders Fogh Rasmussen said on Wednesday the alliance was concerned Russia might push beyond Crimea into eastern Ukraine.
Hagel meanwhile renewed US calls for Russia to de-escalate tensions and restore Ukraine's territorial integrity.
Asked if a steady increase in the number of Russian troops near the Ukrainian border had triggered Hagel's phone call, Kirby said: "I think it was prompted by a number of things, yes, that they kept reinforcing, and that we didn't believe we had a clear indication of intent."
He added that the Pentagon chief was mindful of the Russian military presence in Crimea, the "attack" on a Ukrainian naval base, the death of a Ukrainian officer and concerns raised by Kiev's defense minister.
"It was a lengthy call, lasting about an hour, and I think it's fair to say that at times it was direct," according to Kirby.
The two leaders agreed to keep up a dialogue despite the tensions, he said.
President Barack Obama announced fresh punitive sanctions on Thursday over Moscow's annexation of Crimea, while Russia responded by issuing its own list of sanctions against nine US officials.
Amid the worst East-West crisis since the end of the Cold War, Obama has ruled out the use of military force in Ukraine while vowing to isolate Moscow through diplomacy and sanctions.