The US has 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 on Thursday that 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 .

"We are disturbed by the ongoing violence in eastern Ukraine," White House spokesman Jay Carney said.

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His comments came after Ukraine's acting president said rebels used surface-to-air missiles to shoot down a helicopter. At least 14 military personnel were killed in the incident.

"I just received information that near Slovyansk, the terrorists - using a Russian man-portable air defence system - shot down our helicopter," Oleksandr Turchynov told parliament.

Slovyansk is in the Donetsk region, one of the two provinces in eastern Ukraine that have declared independence from the government in Kiev.

While the US has not been able to verify what happened to the helicopter, Carney said, "We are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside".

Chechen fighters

Even before the incident, Secretary of State John Kerry spoke to his Russian counterpart, Sergey Lavrov, on Wednesday to reiterate US 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.

RELATED: Fear reigns in Donetsk

One of the separatists' leaders made a surprise admission earlier on Thursday that 33 out of more than 40 rebels killed during a raid on a Donetsk airport this week were Russian nationals from Muslim regions such as Chechnya, the AFP news agency reported.

The revelation challenged President Vladimir Putin's rejection of Russian links to the separatist drive and supports Kiev's claims that the rebels do not represent the true will of the population of the eastern regions.

Donetsk airport was the scene of heavy fighting on Monday and Tuesday. The government claims it has taken control of the site, but the separatists have taken the road leading to it and a rebel convoy headed towards the airport on Thursday.

"Clearly that convoy was heavily loaded with weapons and ammunition," Al Jazeera's David Chater, reporting from Donetsk, said. "They are ready for a fight here."

Rebels seized parts of eastern Ukraine in response to the February ouster in Kiev of pro-Kremlin President Viktor Yanukovich.

Chocolate magnate Petro Poroshenko was elected new president of Ukraine last weekend.

Source: Al Jazeera and agencies