(Adds details of call, estimates of Russian troops)
By David Alexander and Phil Stewart
WASHINGTON, March 20 (Reuters) - U.S. Defense Secretary Chuck Hagel received assurances on Thursday from his Russian counterpart that the thousands of Russian troops along Ukraine's eastern border had no plans to enter the country, a Pentagon spokesman said.
Hagel held an often "direct" conversation with Russian Defense Minister Sergei Shoigu about the Ukraine crisis for about an hour, U.S. Rear Admiral John Kirby said.
The Pentagon chief initiated the call, he said, in part because of concerns about the continued reinforcement of Russian troops on Ukraine's eastern and southern borders.
"Secretary Hagel appreciated Minister Shoigu's time and the minister's assurance that the troops he has arrayed along the border are there to conduct exercises only, that they had no intention of crossing the border into Ukraine and that they would take no aggressive action," Kirby said.
Russian forces seized control of Ukraine's Crimea region three weeks ago, touching off the worst East-West crisis since the Cold War and raising fears in the West that Russian President Vladimir Putin might also send troops into eastern Ukraine.
There are tens of thousands of Russian troops massed on the border with Ukraine and the deployments include both tanks and air support, according to a European security source. The forces appear to be left over from recent military exercises that Russia carried out in the region, a second European security source said.
President Barack Obama raised the stakes on Thursday by targeting some of Putin's closest long-time political and business allies with personal sanctions, after Russia formally annexed Crimea this week.
Obama also warned against any further Russian military moves.
"At the same time, the world is watching with grave concern as Russia has positioned its military in a way that could lead to further incursions into southern and eastern Ukraine. For this reason, we've been working closely with our European partners to develop more severe actions that could be taken if Russia continues to escalate the situation," Obama said, announcing the new sanctions.
Ukraine's Ambassador to the United Nations in Geneva told diplomats on Thursday that Russia may be preparing a further military incursion into Ukrainian territory, but Moscow denied that.
Despite his assurances, Defense Minister Shoigu declined to tell Hagel when the Russian military exercises would end.
"The minister said he didn't have a firm timetable on that," Kirby said, adding that Hagel was "clear and firm" in telling Shoigu that because Russian forces had seized control of Crimea, they bore the responsibility for the incidents occurring there.
Hagel was particularly concerned about use of force in Crimea, the attack on the Ukrainian naval base and the killing of a Ukrainian warrant officer, Kirby added. (Reporting by Phil Stewart and David Alexander; editing by G Crosse)