By Phil Stewart
WASHINGTON, March 27 (Reuters) - The United States sees no indications that Russian forces along the border with Ukraine are carrying out the kind of springtime military exercises that Moscow has cited as the reason for their deployment, the Pentagon said on Thursday.
Ukraine's government has put its heavily outnumbered and outgunned forces on alert for an invasion from Russia in the east following Moscow's seizure of Crimea, as the West moves to isolate Russia diplomatically and pressure it economically.
U.S. and European security agencies estimate Russia has deployed military and militia units totaling more than 30,000 people along its border with eastern Ukraine.
Although the Pentagon has cited assurances from Russian Defense Minister Sergei Shoigu that its troops along the border were sent for exercises and that they would not cross into Ukraine, U.S. officials have acknowledged concerns about continued Russian reinforcements to the area.
"We've seen no specific indications that these - that exercises - are taking place," Pentagon spokesman Rear Admiral John Kirby told a news briefing.
But when pressed, Kirby added: "Just because we haven't seen an indication of exercises now doesn't mean that one won't occur."
"(The Russians) made it clear that their intent was to do exercises and not to cross the border. Our expectation is they're going to live up to that word," he said.
British Defense Secretary Philip Hammond, speaking at the Pentagon on Wednesday, appeared to give little weight to any assurances from Shoigu, saying Russian President Vladimir Putin seemed to be calling all the shots.
"Other Russian players, including Minister Shoigu, may express views, but it's a moot point. And we cannot know, we do not know, to what extent all of those people are really inside the inner circle in which President Putin is planning this exercise," Hammond said.
Asked whether Hagel shared Hammond's concerns, Kirby said: "I don't think there's any gap between Minister Hammond's concerns and Secretary Hagel's in that regard."
U.S. and European sources familiar with official reporting, speaking on condition of anonymity, say that U.S. and European government experts believe that there has been, and continues to be, a steady and noticeable buildup in the total number of Russian forces along the Ukrainian border, though some military units have rotated in or out of the area.
The sources said that the Russian force deployed along the Ukraine border includes regular military including infantry as well as armored units and some air support.
Also deployed are militia or special forces units comprised of Russian fighters, wearing uniforms that lack insignia or other identifying markings, similar to the first Russian forces to move into Crimea during Russia's recent military takeover there.
Kirby did not enter into details about what kinds of forces the Russians were fielding but acknowledged broad capability.
"They have them in quite a number and in a composition that provides lots of capability," he said.
The comments came a day after a group of U.S. Republican lawmakers, citing classified intelligence, voiced "urgency and alarm" about Russia's troop buildup and called on President Barack Obama to direct the U.S. military to enhance its alert posture in Europe.
"A failure to take such deterrent actions in the face of continued Russian aggression will certainly risk the very diplomatic and peaceful outcome that we all desire," read the letter, signed by eight members of the House Armed Services Committee, including its chairman, Representative Buck McKeon. (Additional reporting by Mark Hosenball, editing by G Crosse)