Russian Defense Minister Sergei Shoigu promised his US counterpart Chuck Hagel in a telephone call 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.

Hagel also asked how long the military "exercise" would last but Shoigu "didn't have a firm time frame for that," Kirby said.

Washington has watched a Russian buildup on Ukraine's eastern border with growing concern after Moscow's 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 Wednesday the alliance was concerned Russia might push beyond Crimea into eastern Ukraine.

In the phone call, Hagel "pressed" the Russian defense chief to explain "Russian intentions with respect to forces they have aligned near Ukraine's eastern and southern borders," Kirby said.

Hagel also renewed US calls for Russia to de-escalate tensions and restore Ukraine's territorial integrity.

"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.

President Barack Obama announced fresh punitive sanctions earlier 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.