More than 30 pro-Russian separatists were killed in fighting near the rebel stronghold of Slaviansk in eastern Ukraine, Interior Minister Arsen Avakov said on Tuesday.

More than 30 pro-Russian separatists were killed in fighting near the rebel stronghold of Slaviansk in eastern Ukraine, Interior Minister Arsen Avakov said on Tuesday.

"We estimate that the terrorists lost more than 30 people," Avakov wrote on Facebook.

Pro-Russian groups ambushed Ukrainian forces on Monday, triggering fierce fighting near Slaviansk. Avakov said four Ukrainian servicemen had been killed and 20 wounded.

The pro-Russian rebels said on Monday four of their number had been killed in the fighting.

The foreign ministers of Russia and Ukraine met briefly on the sidelines of a Council of Europe conference in Vienna, Austria's foreign ministry said on Tuesday.

"There was a short greeting. They spoke briefly, but not by themselves," a ministry spokesman said of the meeting, late on Monday, between Russian Foreign Minister Sergey Lavrov and Ukrainian Acting Foreign Minister Andriy Deshchytsia.

Austria chairs the Council of Europe human rights group at the moment and is hosting a meeting of 30 foreign ministers on Tuesday to discuss ways to defuse the situation in Ukraine, where the government is trying to quell an insurrection by pro-Russian activists.

Austrian Foreign Minister Sebastian Kurz on Monday played down chances for a breakthrough.

"You cannot expect miracles from the conference because there won't be any, but hopefully, and this is my expectation, there will be clear backing from most countries for orderly and free (presidential) elections on May 25, because this could be a small step towards stabilizing the country," he told Reuters.