REUTERS

The two sides seemed to have reached the final agreement just two days ago, after three-month long negotiations. The Ukrainian side has been working with active involvement of volunteers, the military, the security service (SBU) officers.

Mariupol Court has ruled to downgrade the restrictive measure against militants accused of banditry, robberies and murders from holding in custody to house arrest. This legal technicality allowed taking the criminals agreed for swap to the demarcation line.

Reportedly 13 Ukrainian POWs were supposed to be exchanged for 13 detained militants.

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Among the Ukrainian troops to be freed were officers who had been held captive for over a year after the events near Ilovaisk.

Read alsoPresidential envoy: 245 Ukrainian POWs still in militant captivity in Donbas (Photos)However, at the last moment, the militants suddenly came up with new demands, pushing for the swap to be carried out in the territory of a so-called “DPR”, instead of a neutral zone.

The final-moment talks  on the front line lasted for more than a day but saw no result.

"It was decided to come back and, maybe from Monday, continue the talks to get our guys back," said Yuriy Tandyt, adviser to the SBU chief, in an interview with 112 TV channel.

The Ukrainian negotiators have felt the militants' proposal too dangerous and risky to accept, in order not to jeopradize lives of the Ukrainian POWs, and to make sure they are actually released from captivity, according to Tandyt. 

As UNIAN reported earlier, Vasyl Budyk, former adviser to the deputy defense minister, said that 37 Ukrainian soldiers were being held captive in former building of local SBU.