KIEV, May 29 - Pro-Russian separatists shot down a Ukrainian army helicopter on Thursday, killing 12 soldiers including a general, as government forces pressed ahead with an offensive to crush rebellions in the east.

After weeks of accusations from Kiev of Russian involvement in the uprising, a rebel leader in the eastern city of Donetsk acknowledged that some of his fighters who died in the government offensive had been "volunteers" from Russia, saying their bodies were being returned across the border, Reuters reported.

In Kiev, outgoing acting president Oleksandr Turchynov said the helicopter, which had been carrying supplies in eastern Ukraine, had been brought down by anti-aircraft fire from near the town of Slavyansk, which has been under the control of separatists since early April.

It was one of the heaviest losses suffered by the army during two months of separatist unrest, and followed a fierce assault by government forces in which 50 or so rebels were killed earlier this week.

"I have just received information that terrorists using Russian anti-aircraft missiles shot down our helicopter near Slavyansk. It had been ferrying servicemen for a change of duty," Turchynov told parliament.

The bodies of some of the separatists killed this week when the Ukrainian military fought to regain control of Donetsk international airport were being prepared for return to Russia on Thursday, the rebel leader said.

In an admission that the rebels were being supported by Russian militia fighters, the leader of the self-proclaimed Donetsk People's Republic, Denis Pushylin, said: "Those who are volunteers from Russia will be taken to Russia today."

Interior minister Arsen Avakov accused the government of Russian President Vladimir Putin of being behind the airport violence, which began when rebels seized a terminal the morning after Ukraine's election. Weapons collected at the airport after the rebels were forced out by airstrikes and a paratroop assault had been brought in from Russia, he said.

"These are not our weapons - they were brought from Russia. Serial numbers, year of production, specific models ... I am publishing this photograph as proof of the aggression of the Putin regime," Avakov wrote on his Facebook page.

Kiev's leaders have long asserted that Russia, which annexed Ukraine's Crimea peninsula in March, has fomented the separatist rebellions in the east of Ukraine with a view to bringing about dismemberment of the country. Moscow denies it is involved.

Ukraine's Defense Minister Mikhaylo Koval said on Thursday: "We have put all our forces and equipment into the anti-terrorist operation. We have covered the whole state border." (rt/ez)