A team of international observers abducted by pro-Russian nationalists in eastern Ukraine has been freed.

Separatists kidnapped eight Western observers for the Organisation for Security and Cooperation in Europe in the city of Slovyansk on April 25.

Five Ukrainian military personnel were seized at the same time.

One of the observers was released for health reasons.

The self-declared mayor of Slovyansk, Vyacheslav Ponomarev, confirmed the release of the remaining 12 hostages, and said Russian envoy Vladimir Lukin, who arrived in eastern Ukraine on Friday, had helped negotiate their release.

"As I promised them, we celebrated my birthday yesterday and they left. As I said, they were my guests," Ponomarev said.

He said that there had been no prisoner exchange, and an OCSE delegation was waiting to collect the freed observers on the outskirts of the town.

The OCSE confirmed the release in a tweet.

One of the observers, German Col Axel Schneider, told Associated Press news agency the team had "a very good attitude and that gave them the strength to stand the situation".

Lukin said that he hoped that the "voluntary act" by the separatists would be reciprocated by the Ukrainian government in Kiev. "I would very much like military actions to end," he added.

Ukraine's government confirmed a second day of military operations in eastern Ukraine, with serious fighting reported in Andreevka, 6km east of Slovyansk.