* One of deadliest attacks inflicted on Ukrainian army
* Earlier in week army killed 50 rebels near Donetsk airport
* Some rebel bodies being repatriated to Russia - leader
* One fighter says he's from Abkhazia, others from Chechnya (Adds gas, barricades being moved)
By Sabina Zawadzki and Gabriela Baczynska
DONETSK, Ukraine, May 29 (Reuters) - Pro-Russian separatists shot down a Ukrainian army helicopter on Thursday, killing 14 soldiers including a general, as government forces pressed ahead with an offensive to crush rebellions in the east swiftly following the election of a new president.
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.
In Kiev, outgoing acting president Oleksander Turchinov said the helicopter, which had been carrying supplies in eastern Ukraine, had been brought down by anti-aircraft fire from near the town of Slaviansk, 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 Slaviansk. It had been ferrying servicemen for a change of duty," Turchinov 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 Pushilin, said: "Those who are volunteers from Russia will be taken to Russia today."
At Donetsk's Kalinin morgue, where the dead from the violence were taken, 30 coffins were laid out in rows on Thursday. "Yes. They're going to Russia," said an orthodox priest, who was edgy and did not wish to be named.
In another part of the morgue lay a local man, 43-year-old Mark Zverev, who had also been killed in the airport fighting. "Europe should know what is happening. He's not a terrorist. He is a defender of his home, of his people and of his land," said his mother, clutching his portrait.
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 Defence Minister Mikhailo Koval said on Thursday: "We have put all our forces and equipment into the anti-terrorist operation. We have covered the whole state border."
Russian Foreign Minister Sergei Lavrov accused the West on Wednesday of pushing Ukraine into "the abyss of fratricidal war", and reiterated his call for an end to Kiev's offensive.
A separatist fighter, who gave his name only as Varan and said he was from the breakaway Georgian territory of Abkhazia, said he believed a total of 33 bodies of those killed this week would be taken back to Russia.
Wearing combat fatigues, body armour and reflective sunglasses, Varan told a Reuters correspondent outside Donetsk morgue that the separatists included fighters from Chechnya, Moscow and the southern Russian city of Rostov.
"The number of fighters is increasing and I think that the closer the Ukrainian army gets, the more fighters there will be because, you know, mobilisation has been called," he said.
People in Donetsk, an industrial city of one million where the rebels hold the regional administration building and state security headquarters, said the atmosphere was edgy as rumours circulated that the army was poised to attack.
Asked if he was worried about gunbattles erupting in the city, the prime minister in the self-declared separatist government, Alexander Boroday, said: "A terrorist war may happen in the town although we are doing everything to stop that happening so that a peaceful life can continue in the town."
In front of the provincial administration building, now headquarters of the separatist Donetsk People's Republic, armed men from another separatist group, the "Vostok Battalion", brought heavy lifting machinery to shift barricades. They said they were clearing space for a quick exit in case of attack.
The assault launched on Monday was the first time Kiev has unleashed its full military force against the fighters after weeks of restraint and came the day after Ukrainians overwhelmingly elected Petro Poroshenko as president.
Poroshenko, 48, a billionaire confectionary magnate who became the first Ukrainian since 1991 to win the presidency outright in a single round of voting, marked his victory by calling for a swift offensive to crush the rebellions.
Poroshenko will have an opportunity to meet Putin when both attend commemorations of the 70th anniversary of World War Two's "D-Day" landings in Normandy on June 6, before Poroshenko returns to Kiev for his inuaguration. On June 3, Poroshenko is also expected to meet U.S. President Barack Obama in Warsaw.
The separatist authorities say those who died on Monday and Tuesday included a truckload of wounded fighters blasted apart as they were driven away from the battlefield. The government said it suffered no losses in the operation, when its aircraft strafed the airport and paratroops landed to reclaim it.
A separatist leader in another part of the region acknowledged his men were holding four monitors from the Organization of Security and Cooperation in Europe (OSCE) who went missing in eastern Ukraine on Monday.
Vyacheslav Ponomaryov, whose group controls the town of Slaviansk, said the OSCE had been warned not to travel in the area, but had sent a four-man team all the same. He said they would be released soon.
The OSCE sent in about 300 observers to monitor compliance of an international accord for de-escalating the crisis in eastern Ukraine, where separatists have seized control of strategic points in several towns.
In Berlin, talks between Russia, Ukraine and the European Commission to resolve a gas dispute were to go ahead on Friday, the Commission said, as time was running out to avert a threat of Moscow cutting off supplies to Ukraine.
Much of the gas Russia sells to the EU passes through Ukraine, so the dispute threatens onward supplies to Europe.
Moscow is urging Ukraine to pay part of its outstanding debt of more than $5 billion for gas supplied since last November. If Kiev fails to pay, Russia says it will continue supplies only on conditions of pre-payment. Ukraine says it will not make any payments until the two sides agree a new price for gas for 2014. (Additional reporting by Lina Kushch in Donetsk and Natalya Zinets and Gareth Jones in Kiev; Writing by Richard Balmforth; Editing by Peter Graff)
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