by Griff Witte

and Michael Birnbaum

Pro-Russian separatists on Thursday dealt a heavy blow to Ukrainian military efforts to win back the rebellious east, even as tensions between insurgent factions spilled into the open in this anxious regional capital’s downtown.

The blow came when separatists wielding a shoulder-mounted rocket launcher scored a direct strike against a Ukrainian military helicopter ferrying a senior commander of the government’s counterterrorism operation.

Ukraine’s acting president, Oleksandr Turchynov, confirmed in a speech to parliament that the transport helicopter was downed. He said 14 military personnel aboard were killed, including Serhiy Kulchytskiy, a general who had served in the Soviet army and was in charge of combat training for Ukraine’s national guard.

1 of 29 Full Screen Autoplay Close May 31, 2014 Saturday May 29, 2014 Thursday Skip Ad × Ukrainian government vows more military action View Photos The Ukrainian government vowed to push ahead with military operations against pro-Russian separatists in the embattled east of the country after using attack helicopters and fighter jets to retake Donetsk’s airport. Caption The Ukrainian government pledged to push ahead with military operations against pro-Russian separatists in the embattled east of the country after using attack helicopters and fighter jets to retake Donetsk’s airport. Pro-Russian members of the Vostok militia battalion run a training exercise at their base near Donetsk, in eastern Ukraine, where clashes between pro-Russian forces and Ukrainian troops have persisted. Valentina Svistunova/European Pressphoto Agency Buy Photo Wait 1 second to continue.

Turchynov said “terrorists” brought down the helicopter with a shoulder-launched heat-seeking missile, which he suggested had been supplied by Russia.

The attack coincided with apparent infighting between eastern Ukraine’s separatists that could signal an even deeper role for Russia in the conflict.

The Vostok battalion — a militia known to include Chechen and other Russian fighters that has only recently emerged as a major force — took over the headquarters of the self-declared Donetsk People’s Republic in the heart of this city of 1 million on Thursday.

A video posted on a Donetsk news Web site showed men in camouflage setting up heavy weaponry outside the seized building that serves as the separatist headquarters. Officials with the Donetsk People’s Republic, the group that originally overran the building last month, streamed through the exits, with some expressing surprise and dismay at being forced to leave by putative allies.

Once the building had been cleared, heavily armed Vostok forces brought in backhoes and dump trucks to clear the makeshift barricades that have ringed the building for nearly two months. They said they planned to install more-professional defenses. They also showed off what they said were looted goods from a local grocery store that they found inside the building.

A spokeswoman for the Donetsk People’s Republic later denied that there was any conflict among the separatists and said the forces were simply working to weed out criminals.

“We are keeping order in the building,” said Klavdia Kulbatskaya, the spokeswoman. “Our authorities are in control of the situation. Soon we will obtain order in the premises. We don’t want to be associated with thugs and marauders.”

But there have been divisions in the rebel movement for some time, and varying degrees of Russian influence may be a crucial source of tension. Although both Ukraine and the West have accused Russia of orchestrating insurrection in the east, some of the rebel factions have at times openly defied Moscow’s publicly stated will.

The apparent rise of the Vostok battalion may reflect a bid by Russia to reassert its authority.

The group is thought to have helped orchestrate an attack this week on the Donetsk airport, which prompted a furious response from the Ukrainian military that left scores of rebels dead.

In a sign of Russia’s deep involvement in the fighting here, the country’s state-run RIA Novosti news service reported Thursday that 33 of those killed Monday were Russian nationals.

“All of those dead have been identified,” said Alexander Borodai, who calls himself the prime minister of the Donetsk People’s Republic, according to RIA Novosti. Russian news media reported that the bodies would soon be returned to Russia.

It was unclear how many of the dead fighters were Ukrainian. RIA Novosti reported that more than 100 people were killed, including civilians.

U.S. Secretary of State John F. Kerry pressed his Russian counterpart, Sergei Lavrov, to “end all support for separatists, denounce their actions and call on them to lay down their arms,” State Department spokeswoman Jen Psaki said Thursday.

But the Russian Foreign Ministry, which has denied any Russian government involvement in the insurgency, blamed the violence on Ukraine and called on the Ukrainian military to end its “fratricidal war.”

The turmoil continued Thursday with the downing of the helicopter amid heavy fighting in Slovyansk, a city about 100 miles west of the Russian border that has been the scene of recent clashes between separatists and government troops.

The chopper was flying Ukrainian troops to positions set up on a hill outside the city, Turchynov told parliament.

“Our armed forces, our security forces will complete their job against terrorism,” he said. “And all the criminals who are now funded by the Russian Federation will be destroyed or sit in the dock.”

The emotional speech was followed by a minute of silence in parliament.

An Associated Press reporter saw the helicopter go down with a trail of black smoke and heard gunshots near the crash site as a Ukrainian air force jet circled overhead, the news agency said.

A video posted online appeared to show the attack, with the helicopter spiraling downward in flames.

The Ukrainian national guard said in a statement that the Mi-8 transport helicopter was fired upon from a forest.

The statement said 12 people were killed, compared with the 14 announced by Turchynov. The discrepancy in the death toll was not immediately explained.

After the attack, the military used artillery and fighter jets to bombard the area from which the missile was fired, “destroying” a group of “criminals,” the national guard said.

As the fighting raged, Vyacheslav Ponomariov, the self-

proclaimed “people’s mayor” of Slovyansk, told Russia’s Interfax news agency that his group was holding four election monitors from the Organization for Security and Cooperation in Europe who disappeared Monday.

The four — a Dane, an Estonian, a Swiss and a Turk — were “fine,” and his group was in negotiations with the OSCE to release them, Ponomariov said.

The group abducted another team of OSCE monitors last month.

He said that his men had advised OSCE monitors not to travel across the embattled east but that the captured monitors had been the “most zealous” ones.

Other monitors have been detained, held for several hours and then released. Separatists, who have seized government buildings and declared sovereign republics in two eastern regions, prevented voting in most of eastern Ukraine in Sunday’s presidential election.

Birnbaum reported from Kiev. Abigail Hauslohner in Moscow, Alex Ryabchyn in Kiev and Daniela Deane in London contributed to this report.