Kiev and pro-Russian rebels reached an agreement on Friday to pull back troops, swap hostages and send humanitarian aid to war-ravaged eastern Ukraine, an OSCE official said.

Organisation for Security and Cooperation in Europe representative Heidi Tagliavini told reporters that "a number of agreements" had been reached after five hours of talks.

"Among them are a ceasefire, access to humanitarian aid, troop withdrawal and an all-for-all exchange of detained people," she said, without giving further details.

The crucial issue of the future status of self-proclaimed rebel entities the Donetsk People's Republic and Lugansk People's Republic was not discussed, she added.

The deal hammered out in Minsk, Belarus, comes in the wake of a lightning rebel counter-offensive in southeastern Ukraine that Western powers say was spearheaded by regular Russian troops.

Ukrainian President Petro Poroshenko welcomed the agreement and said he expects to release hostages soon, "most probably tomorrow".

"The fact that this ceasefire should last long is now our common responsibility," he said, speaking in English.

"I am very satisfied," he said, adding that he hoped the agreement would be "the basis for finding a peaceful solution".

Mikhail Zurabov, Russia's ambassador to Ukraine, said an expert group would be formed over the next three days to hammer out a plan to exchange hostages.

"There are more than a thousand detainees on each side," Moscow's ambassador to Ukraine said.

He added that Russia planned to send a new humanitarian convoy to Ukraine shortly.

Kiev's representative at the talks, former president Leonid Kuchma, said the two sides had "exchanged a list of hostages and are starting the process immediately".

"There is no other way except peace. Enough of fighting," he added.