DONETSK, Ukraine (AP) - A Ukrainian military helicopter was shot down by rebels Thursday over Slovyansk amid heavy fighting around the insurgent-held city in eastern Ukraine.

Slovyansk has become the epicenter of fighting between pro-Russia insurgents and government forces in recent weeks. The city - located 100 miles (160 kilometers) west of the Russian border - has seen constant clashes and its residential areas have regularly come under mortar shelling from government forces, prompting some residents to flee.

An Associated Press reporter witnessed the helicopter's downing Thursday. It wasn't immediately clear what weapons the rebels used to shoot it down, exactly where the helicopter fell or what happened to its crew.

The Kiev government condemns the insurgency roiling the east as the work of "terrorists" bent on destroying the country and blames Russia for fomenting it. Russia 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. Still, fighters from Russia, including the battled-hardened region of Chechnya, have been appearing recently in the ranks of the separatists.

Also Thursday, an insurgent leader in eastern Ukraine said his fighters are holding four observers from the Organization for Security and Cooperation in Europe and promised that they would be released imminently.

Vyacheslav Ponomarev, the self-proclaimed "people's mayor" of Slovyansk, a city in the Donetsk region, told The Associated Press 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 said, without setting any specific timeframe.

The OSCE said it had lost contact with one of its four-man monitoring teams in Donetsk on Monday evening. Rebels have previously kidnapped military observers working under the auspices of the OSCE.

The OSCE monitors have been deployed to Ukraine to monitor security situation following Russia's annexation of Crimea and a pro-Russia separatist insurgency that has engulfed regions in eastern Ukraine. They also observed Sunday's presidential vote, won by billionaire candy magnate Petro Poroshenko.

Poroshenko has promised to negotiate with people in the east, where insurgents have seized government buildings and fought government troops for a month-and-half. But he also vowed to continue a military operation to uproot the armed rebels and bring it to a quick end.

In the most ferocious battle yet, rebels in Donetsk tried to take control of its airport Monday but were repelled by Ukrainian forces using combat jets and helicopter gunships. Dozens of men were killed and some morgues were overflowing Tuesday. Some insurgent leaders said up to 100 fighters may have been killed.

The mood in Donetsk was calm Thursday, although many businesses have stopped opening their doors over fear of renewed fighting.

The rebels have declared the Donetsk and Luhansk regions independent of Ukraine. They 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 supported an OSCE peace plan that calls for ending hostilities and launching a political dialogue. Russia also said it would be ready to work with new leader Poroshenko, but strongly urged the Ukrainian government to end its military operation in the east.

Chechnya's Moscow-backed leader, Ramzan Kadyrov, on Wednesday rejected allegations he had sent his paramilitary forces to Ukraine, but said he can't stop fellow Chechens acting on their own from joining the fight.