President-elect pledges to do everything to stop further deaths after attack over Slavyansk kills 14, including army general

Ukraine's president-elect, Petro Poroshenko, has vowed to punish pro-Russian rebels who shot down an army helicopter in the east of the country, killing 14 people, including a general, in one of the deadliest attacks of the insurgency.

The Mil Mi-8 helicopter gunship was downed over Slavyansk by a sophisticated surface-to-air missile on Thursday, prompting the White House to say the incident raised concerns about the rebels being supplied "from the outside".

"We have to do everything we can to ensure no more Ukrainians die at the hands of terrorists and bandits," said Poroshenko, according to Ukrainian news agencies. "These criminal acts by the enemies of the Ukrainian people will not go unpunished."

The Ukrainian defence minister, Mykhailo Koval, is due to give details on the attack at a press briefing on Friday.

The incident was part of continuing bitter and bloody resistance against government forces in eastern Ukraine on Thursday as Russian volunteer fighters moved to exert greater control over the rebel movement.

Sporadic fighting has continued in Slavyansk following a battle for Donetsk airport last weekend in which at least 50 rebels were killed. The escalation in violence seems to have hardened local opposition to Kiev's "anti-terrorist operation" to retake control of the Donbass, which includes the Luhansk and Donetsk regions.

Rebels also attacked a national guard base in the Luhansk region on Thursday, local news outlet Ostrov reported.

The government has condemned the insurgency in eastern Ukraine as the work of "terrorists" bent on destroying the country, and blames Russia for fomenting it. Moscow denies the accusations, saying it has no influence over rebels, who insist they are only protecting the interests of the Russian-speaking population of the east.

But a group of largely Russian fighters, known as the Vostok (East) battalion, has come to the head of the resistance since the airport battle and on Thursday began to bring its own order to the rebel movement. The unit includes fighters from the battle-hardened region of Chechnya, recently annexed Crimea, and other parts of Russia and Ukraine. It played a large role in the battle for the airport, a battalion member, who identified himself by his nickname Ram, told the Guardian.

Armed and masked men from the Vostok battalion took control of the Donetsk regional administration building on Thursday, the heart of the insurgency where pro-Russian protesters declared a "Donetsk People's Republic" in April. After a tense standoff during which snipers appeared briefly on the roof of a nearby apartment building, the Vostok men began to clear out the building, the different floors of which have previously been occupied by various rebel groups. On their command, bulldozers began to remove barricades outside made up largely of tyres, paving stones and barbed wire.

One of the battalion's commanders, who identified himself only by his nickname Ros, said the barricades were no longer necessary and posed a fire hazard. He said the building would be cleared of various rebel groups so officials of the Donetsk People's Republic could continue their protest.

"Why are there so many groups? We are one country," he said, referring to the Donetsk People's Republic.

Roman Romanenko, a former paratrooper and coal miner who was appointed to head the defence of the building in April, said no rebels would live in the building from now on. Both he and Ros denied there was a power struggle between the Vostok battalion and the Donbass people's militia.

People's republic spokeswoman Klavdia Kulbatskaya said the battalion men were "checking" rebels from the people's militia who had been accused of looting. This followed a trend of Russian commanders implementing harsh order among the rebels in recent days. Specifically, two rebels were shot earlier this week in Slavyansk for "looting, armed robbery, kidnapping, leaving battle positions", at the command of Igor Strelkov, a Russian-born leader whom Kiev authorities accuse of being an agent of the Russian intelligence services.

Ros said the battalion had let the looters go and that "the people will deal with them".

Mark Galeotti, a New York University professor who studies the Russian security services, wrote on his blog this week that the appearance of the Vostok battalion, which has its roots in an erstwhile Chechen unit that fought in the 2008 Georgian war, was a move by Moscow to "reverse the slide towards war-lordism" in eastern Ukraine and exert more control over the uprising it has encouraged. Vladimir Putin has indicated Moscow is ready to negotiate with Poroshenko and the new leadership in Kiev.

The deaths of civilians in east Ukraine has further inflamed hatred toward the Kiev government, which many people already view with deep mistrust. A group of residents gathered outside the Kalinin morgue in Donetsk on Thursday to take the body of Mark Zveryev, a 43-year-old taxi driver killed alongside pro-Russian forces near the airport on Monday, to a cemetery to be buried. Tatiana Kozodavenko, a nursery school teacher who previously taught Zveryev's stepdaughter, said anger was growing among the population.

"First there was bewilderment and disbelief, but it's now turning into anger," she said.

Outside the Donetsk administration building, Rostislav, a telecommunications employee whose company closed last month, said his friend "took up a machine gun" with the rebels after a shell hit his home during the fighting in Donetsk on Monday.

"If the Ukrainian military's aggression continues, many people will join the rebellion," Rostislav said. "If my friends fighting with the rebels are hurt, or if my home is damaged, I won't run away, I'll take up weapons and join."