Donetsk, Ukraine (CNN) -- At least 14 people, including a general, were killed Thursday when a Ukrainian military helicopter was shot down by "terrorists" near Slovyansk, acting President Oleksandr Turchynov told Parliament.
Turchynov said the chopper, which was carrying soldiers for a troop rotation, was shot down with a Russian rocket-propelled grenade launcher.
Militants in the rebel stronghold claimed responsibility for downing the helicopter, a spokesman for the militants said. The aircraft had flown there from Kramatorsk, he said, where the Ukrainian military has a substantial presence.
The large loss of life will be a major blow to the Ukrainian military, which on Monday toughened its approach to the separatist movement when it launched a fierce assault on militants who'd taken control of part of Donetsk airport.
Also in Slovyansk, the self-declared mayor said pro-Russia separatist militants are holding four European observers who have been missing since Monday. But he declined to say where.
"Our militants got them," Vyacheslav Ponomarev said Thursday. "They were detained because they didn't respect my request. I asked them not to leave Donetsk (city). They decided they were smarter and could come here."
Ponomarev added that the monitors from the Organization for Security and Co-operation in Europe were not being held in the town.
They also are not "exchange material," he said, indicating that they were not being held to swap for separatists detained by Kiev authorities.
Ponomarev said he had been in contact with the OSCE, and they were assessing the situation. He said he would probably release the monitors soon but wouldn't say when.
On Wednesday, Ukrainian Foreign Ministry spokesman Yevhen Perebynis said that a pro-Russia group was holding the monitors and that negotiations for their release were "in process."
The four team members, who are Swiss, Turkish, Estonian and Danish, were on a routine patrol Monday east of Donetsk city when last heard from, according to the OSCE.
The last time an OSCE team went missing in the Donetsk region, its members also turned up in the hands of the militant separatists in Slovyansk and were described by Ponomarev as "prisoners of war." They were freed just over a week later.
There were fears Wednesday for the safety of another group of 11 monitors who went missing after being stopped at a roadblock in Marinka, west of Donetsk city, but the group later re-established contact with the OSCE.
The OSCE said Thursday that the 11 had been abducted for seven hours by a dozen-strong armed group but were released.
Separatists: Bodies to be returned to Russia
Dozens of pro-Russia separatists were killed after Ukrainian security forces launched their assault on Donetsk International Airport on Monday, following the militants' seizure of a terminal.
The military's move -- only hours after President-elect Petro Poroshenko declared his election win -- was interpreted by some as an indication that he will take a tougher stance as he seeks to unify the country.
Now, the grim task of returning the dead to their families is under way.
At least 33 Russian citizens are among the militants killed during the violence, a spokesman for the self-declared Donetsk People's Republic told a CNN team at the morgue.
The bodies have been identified, and there is an agreement with the local police to escort the remains back to Russia soon, several separatist representatives said.
The bodies will be taken across the border to the southwestern Russian city of Rostov-on-Don. From there, they will be transported to different cities according to where they are from, reportedly including Moscow and Grozny in the Russian republic of Chechnya.
The CNN team at the morgue in Donetsk city saw at least 10 coffins.
The separatist spokesman at the morgue said the men were volunteers from across Russia who had come to help stand with the separatists against the authorities in Kiev.
The spokesman also said that there are still about 15 bodies left at the airport that the separatists haven't been able to collect or identify. About 20 of those killed are Ukrainian, he said.
The Donetsk mayor's official website put the airport death toll at 40 Tuesday, including two civilians, but did not specify how many were separatists.
But the separatist spokesman said the death toll could be as high as 70.
If confirmed, this would make Monday the deadliest day in Ukraine since the bloody clashes in Kiev's Independence Square, or Maidan, which triggered the ouster of pro-Moscow President Viktor Yanukovych in February and led to the current spiral into violence in the east.
The separatists have until now insisted that their movement is homegrown, so their very public acknowledgment of so many Russian citizens among those killed Monday marks a shift in approach.
It could be intended to goad Moscow into a response at a time when the separatist movement is under pressure to find a way forward and has shown signs of division within its ranks.
Kiev and the West have accused Russia of coordinating and supplying the separatists, a claim Moscow has denied. Russia has said it will respect Sunday's election results.
On Thursday, a U.S. official with direct knowledge of the latest intelligence said 30,000 Russian troops that had been amassed along the Russia-Ukraine border have been moving back to their home bases over the last several days. About 10,000 troops -- mainly infantry -- remain along Russia's border with eastern Ukraine, the official said, and there are some signs those troops will be pulling back as well.
NATO this week also reported signs that some Russian troops near Ukraine's eastern border may be preparing to pull back.
The United States has been using satellites to track Russian troop movements for weeks, and both the United States and NATO have released imagery to bolster their case that Russia had amassed tens of thousands of troops along the border.
Several U.S. officials have said it appears the Russians have decided to try to work with the new Ukrainian government, but officials also point out that it wouldn't take much to bring a large number of Russian forces back to the border via transport aircraft at any time.
Governor in talks with separatists
Serhiy Taruta, the billionaire governor of Donetsk, acknowledged in a CNN interview in Kiev that a Ukrainian military operation was under way in the Donetsk region, but he was unable to comment further.
Taruta said he was in talks with pro-Russia separatist leaders in the region to try to calm the situation, adding that the aim was to "reach real understanding," rather than just a deal.
"We are having dialogue, which is dragging out because they seem to have a constant rotation of their leaders, but I hope that in the final analysis we will reach an agreement and reconciliation to work together," he said.
Taruta has met with Poroshenko in Kiev, he said, adding that the President-elect will visit the Donetsk region "we hope in the first half of June."
Meanwhile, a spokeswoman for the Donetsk People's Republic said that some barricades were being cleared away from in front of the regional administration building in Donetsk city that the separatists have made their headquarters for weeks.
She said that separatists were "cleaning up" and "instilling order" and that a news conference would soon explain more.
Earlier, militants surrounded the building and said they were searching it for looters but explained little more.
Clashes in eastern Luhansk
Amid the ongoing tensions, Ukraine's National Guard base in the eastern Luhansk region was attacked Wednesday by what the country's Interior Ministry described as "terrorists."
"There have been losses among military personnel as well as among the attackers," a statement from the ministry said.
The Luhansk region is, with neighboring Donetsk, at the heart of the separatists' bid to declare independence from Ukraine.
Eastern Ukraine was a key support base for Yanukovych, and many people there oppose the authorities in Kiev, favoring closer ties instead to Russia.
CNN's Nick Paton Walsh reported from Donetsk, and Laura Smith-Spark wrote and reported in London. CNN's Barbara Starr, Hala Gorani and Andrew Carey contributed to this report.