Darko Vojinovic, Associated Press
Pro-Russian rebel leader from eastern Ukraine Andrei Purgin smiles during a press conference in the town of Donetsk, eastern Ukraine, Friday, Sept. 12, 2014. Government and rebel forces early Friday exchanged dozens of prisoners captured during fighting in Ukraine, as part of a cease-fire agreement sealed earlier this month. The transfer took place in the dark outside the main rebel stronghold of Donetsk under the watch of international observers.

DONETSK, Ukraine — In the dead of night, Ukrainian troops and Russian-backed rebel forces on Friday exchanged dozens of prisoners who had been captured during fighting in eastern Ukraine, part of a cease-fire deal that has struggled to succeed.

The transfer took place in the dark outside of the main rebel stronghold of Donetsk under the watch of international observers.

Thirty-six Ukrainian servicemen were released after negotiations, Ukrainian President Petro Poroshenko said. Ukrainian forces handed over 31 pro-Russian rebels detained over the five-month conflict, some of them Russian citizens.

The cease-fire took effect a week ago but has been routinely violated. Shortly after the prisoner exchange, a volley of rocket fire was heard in Donetsk.

The group of Ukrainian servicemen was driven away from local rebel headquarters around 1:30 a.m. and taken several miles north of Donetsk, where they were met by Ukrainian military officials.

The two sets of captives were brought out wearing handcuffs, which were removed as they were handed over. One representative from each side checked each prisoner against a list and crossed out their name as they were freed.

Hundreds of prisoners are believed to be held by both sides, although no firm figure has been provided by either.

"There is an ongoing process of talks. We are meeting each other's demands and fulfilling our promises," said Yuriy Tandit, a negotiator for the government.

Darya Morozova, who is overseeing the prisoner exchange for the separatists, said she estimates around 1,200 rebels and their supporters are being detained by Ukrainian authorities. She said the rebels were holding several hundred Ukrainian troops, but when asked for an exact figure, she would only say it was "up to 1,000" people.

Morozova claimed the rebel prisoners had been poorly treated and some had not been fed for around two weeks. Another transfer of prisoners is expected in the next three days, she said.

Some of the separatists freed Friday were Russian citizens.

One of them, Simon Veridya from Moscow, said he was captured in the town of Kramatorsk, which was retaken by government forces in July.

"They shot at our ambulance. There were five of us, including two women. We were taken to custody in Kramatorsk at the antiterrorist operation base at the airport," Veridya said. "I was beaten and have two broken ribs."

The conflict between Russian-backed rebels and the Ukrainian government has been raging since mid-April, claiming more than 3,000 lives, according to the UN. Hundreds of thousands have been forced to flee the fighting.

Ukraine and the West have accused Russia of propping up the insurgency in eastern Ukraine with recruits and heavy weapons. Moscow has admitted that Russian volunteers were fighting across the border but denied sending the rebels weapons or troops.

In Brussels, the European Union imposed a new round of sanctions on Russia for its actions in Ukraine, virtually shutting down long-term lending for several Russian weapons manufacturers as well as three energy companies including Rosneft, the country's largest oil producer. EU entities and individuals will now be barred from providing lending to those firms for more than 30 days.

Laura Mills in Kiev, Ukraine, and Nataliya Vasilyeva in Moscow contributed to this report.