The volunteer group and representatives of the self-proclaimed Donetsk People's Republic continue talks on a swap of prisoners-of-war (POWs), adviser to the Ukrainian Security Council (SBU) head Yuri Tandit told Interfax on Tuesday.

He said that a 13-for-13 captive exchange had been planned at the end of the previous week, but it had been disrupted and the DPR was to blame.

"The talks have been held by a volunteer group, particularly representative Vasyl Kovalchuk, a fighter-internationalist, he has already released our guys several times. He has held these talks for a few months, we have been preparing for a military operation," Tandit said.

"On Friday, people who have been spoken of by the Donetsk side were taken away under Alfa's convoy, but we realized that we do not have enough time to do it, as it was happening at night and this poses some threats. The shelling started and the release was delayed for the next day," he said.

"Already the next morning when Donetsk began to change the terms of the talks - I am telling you about this citing Vasyl Kovalchuk - they actually told us that what was promised [by Donetsk] is not being fulfilled. We allegedly should have replaced those people who were at first on the list given to us for their release. And we decided to suspend this operation so as to resume the talks on Monday. Now the group is working in the city of Donetsk and I hope that a group of our heroes will be freed in the next few days," Tandit said.

Meanwhile, he said that the latest figures on the number of captives, putting it at precisely 172, are constantly on the decline.

"The process of calculating freed people is very difficult. Therefore, new data is coming to the center and the SBU is analyzing the facts which are being reported by the Interior Ministry, Defense Ministry and volunteer battalions. There are some places where our guys are getting released not only as a result of military operations, but also those who were led out by the patriots," the adviser to the SBU chief said.