* 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
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, 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 tried 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."
Interior minister Arsen Avakov accused the government of Russian President Vladimir Putin of being behind the airport violence. 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 this but they also allege that it is failing to stop Russian fighters from crossing the long land border into Ukraine together with truckloads of guns and live ammunition.
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 separatist "prime minister" 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."
The assault launched last 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.
Though he is unlikely to be inaugurated before June 7, 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. On June 3, Poroshenko is also expected to have talks with 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.
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.
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 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.