Acting President Oleksandr Turchynov told the parliament in Kiev that rebels used a portable air defense missile to bring down the helicopter. He said 14 died, including Gen. Serhiy Kulchytskiy, according to the Interfax news agency.
Ukraine's National Guards put the death toll at 12 including the general, but said information about the crash was still being clarified. It said one soldier was badly wounded.
Slovyansk, a city of 120,000 residents 100 miles (160 kilometers) from the Russian border, has become the epicenter of fighting between pro-Russia insurgents and government forces in recent weeks. It's in the Donetsk region, one of the two provinces in eastern Ukraine that have declared independence from the central government in Kiev.
An Associated Press reporter saw the helicopter go down amid a trail of black smoke. Gunshots were heard in Slovyansk near the crash site and a Ukrainian air force jet was seen circling above. It was too dangerous to visit the site itself.
Turchynov said the helicopter was rotating troops into a checkpoint when it came under rebel fire. Residential areas in Slovyansk have regularly come under mortar shelling from government positions on a hill above the city, causing civilian casualties and prompting some residents to flee.
Interfax said Kulchytskiy had once served in the Soviet army and was in charge of combat training for Ukraine's National Guards.
The Kiev government condemns the insurgency that is roiling the east as the work of "terrorists" bent on destroying the country and accuses Russia of fomenting it. Russia denies the accusations, saying it has no influence over rebels, who insist they are only protecting the interests of Russian-speakers in the east.
Billionaire candy magnate Petro Poroshenko, who just won Ukraine's presidential vote, has promised to negotiate with people in the east but has also vowed to uproot the armed rebels. He is not yet in charge however — a date is still being set for his swearing-in early next month — and has not spoken about the insurgency since his victory speech Monday.
Russia's Foreign Ministry on Thursday denounced the use of aircraft and artillery against the rebels and demanded that Kiev end a "fratricidal war and launch a real political dialogue with all political forces and representatives of the regions."
The ministry said it would be impossible to restore peace in Ukraine without ending the government's military action against the rebels and withdrawing Ukrainian troops from the east. It called on the West to use its clout with Kiev to "stop Ukraine from sliding into a national catastrophe."
The separatists in Ukraine have pleaded to join Russia, but President Vladimir Putin has ignored their appeal in an apparent bid to de-escalate tensions with the West and avoid a new round of Western sanctions.
Putin has denied sending any troops into Ukraine, but fighters from Russia, including from the battle-hardened region of Chechnya, have been appearing among the separatists.
An insurgent in Donetsk, who identified himself only by his nom de guerre, Baran (Ram), told reporters Thursday that 33 Russian citizens were among the rebels killed earlier this week.
Dozens of rebels were killed — insurgent leaders said the death toll might be up to 100 fighters — when Ukrainian forces used combat jets and helicopter gunships Monday to dislodge those trying to seize control of Donetsk airport, Ukraine's second-largest.
The mood in Donetsk was calm Thursday, although many businesses have stopped opening due to fears of renewed fighting following Monday's ferocious battle.
An insurgent leader, meanwhile, confirmed that his fighters were holding four missing observers from the Organization for Security and Cooperation in Europe and promised they would be released shortly. Vyacheslav Ponomarev, the self-proclaimed "people's mayor" of Slovyansk, told the AP that the monitors — who are from Turkey, Switzerland, Estonia and Denmark — were safe.
"I addressed the OSCE mission to warn them that their people should not over the coming week travel in areas under our control. And they decided to show up anyway," Ponomarev said. "We will deal with this and then release them."
He did not give a specific timeframe.
The OSCE had lost contact with the team in Donetsk on Monday evening. Their teams are in Ukraine to monitor security situation following Russia's annexation of Crimea in March and the rise of the pro-Russia separatist insurgency.
Leonard reported from Donetsk, Ukraine. Laura Mills in Kiev and Vladimir Isachenkov in Moscow contributed to this report.
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