An insurgent leader said Thursday that his fighters are holding four observers from the Organization for Security and Cooperation in Europe.

Vyacheslav Ponomarev, the self-proclaimed "people's mayor" of Slovyansk, a city in the Donetsk region, said the four monitors are safe and promised that they would be freed soon, according to the Interfax news agency. He didn't set any conditions for their release.

The OSCE said it had lost contact with one of its four-man monitoring teams in Donetsk on Monday evening. Rebels have previously kidnapped OSCE monitors in Ukraine.

Ponomarev said that his men had advised OSCE monitors not to travel across the region, but "there were those four most-zealous ones, and they were detained."

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 furious 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.