The US expressed new concerns about rising violence in eastern Ukraine, including the downing of a military helicopter by pro-Russian rebels battling the government.
The White House and State Department both said a de-escalation of the crisis was imperative.
They 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 US 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 yesterday that 12 troops died when rebels shot down a military helicopter in Slovyansk using a portable air defence missile.
Even before the incident, Secretary of State John Kerry had spoken to his Russian counterpart, Sergey Lavrov, to reiterate US concerns about the deteriorating situation in Ukraine, the State Department said.
Mr Kerry raised with Mr Lavrov reports of Chechen fighters crossing into Ukraine to join the separatists, spokeswoman Jen Psaki said.
Mr Carney and Ms Psaki also said it was unacceptable that insurgents have detained four observers from the Organisation for Security and Cooperation in Europe. They demanded their immediate release.
The US has called on Russia repeatedly to help de-escalate tensions in Ukraine, including withdrawing troops massed near Ukraine.
Defence secretary Chuck Hagel said yesterday 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.”
Mr 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,” Mr Hagel said.
He spoke with reporters travelling with him at the start of a 12-day trip to Asia and Europe.
PA