The White House and State Department both said a de-escalation of the crisis was imperative and called on Russia to exert pressure on the separatists to get them to end the fighting and release a group of international monitors who have been detained in eastern Ukraine since earlier this week.
"We are disturbed by the ongoing violence in eastern Ukraine," White House spokesman Jay Carney said. While the U.S. has not been able to verify what happened to the helicopter, he said, "We are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside."
Ukraine's acting president said earlier Thursday that 12 troops died when rebels shot down a military helicopter in Slovyansk using a portable air defense missile.
Even before the incident, Secretary of State John Kerry spoke to his Russian counterpart, Sergey Lavrov, on Wednesday to reiterate U.S. concerns about the deteriorating situation in Ukraine, the State Department said.
Kerry raised with Lavrov reports of Chechen fighters crossing into Ukraine to join the separatists, spokeswoman Jen Psaki said.
Kerry "pressed Foreign Minister Lavrov to end all support for separatists, denounce their actions and call on them to lay down their arms," she said.
"Our broad view, as you know, is that de-escalation is the proper path forward," Psaki added, although she said she was not aware of concerns that Ukrainian security forces were using disproportionate means to quell the fighting as some Russians have alleged.
Carney and Psaki also said it was unacceptable that insurgents have detained four observers from the Organization for Security and Cooperation in Europe. They demanded their immediate release.
The U.S. has called on Russia repeatedly to help de-escalate tensions in Ukraine, including withdrawing troops massed near Ukraine.
Defense Secretary Chuck Hagel said Thursday the departure of thousands of Russian troops from the Ukraine border was "promising," but he added, "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 he has not spoken to his Russian counterpart about the withdrawal.
"We do know that thousands of Russian troops have been pulled back and are moving away. But we also know that there are still thousands of Russian troops still there that have not yet moved," Hagel said.
Hagel spoke with reporters traveling with him at the start of a 12-day trip to Asia and Europe.
Associated Press writer Josh Lederman contributed to this report.