Russia Pulls Back Most Troops But Many Remain

American defense officials have confirmed Russia has pulled most of its forces away from Ukraine's border. But there is still more work to be done, especially to calm tensions in Eastern Ukraine.

Defense Secretary Chuck Hagel told reporters thousands of Russian troops have been pulled back, but many thousands still remain.

"They are not where they need to be and won't be until all of their troops that they positioned along that border a couple of months ago are gone," Hagel said.

"We do know that thousands of Russian troops have been pulled back and are moving away," he continued. "But we also know that there are still thousands of Russian troops still there that have not yet moved."

Meanwhile, Ukraine's acting president said at least 12 troops died when rebels shot down a military helicopter in Slovyansk.

The White House continues to call on Russia to push separatists to end the fighting and to release a group of international monitors who have been detained.

"We are disturbed by the ongoing violence in eastern Ukraine," presidential spokesman Jay Carney said at a White House news conference.

"We are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside," he said.