Militants have released 964 Ukrainian soldiers taken captive in southeastern Ukraine, director of the Ukrainian Security Service (SBU) Valentyn Nalyvaichenko told the Channel 5 television station on Thursday.

"We have managed to release 964 people from terrorists and Russian special services," he said.

Another 684 Ukrainian servicemen are still being held by militants, and 1,664 remain listed as missing, Nalyvaichenko said.

"We insist on an all-for-all exchange of POWs," he said.

Head of the SBU Central Investigative Department Vasyl Vovk said at a press conference in Kyiv on Wednesday that 684 Ukrainian soldiers were being held by Russian-supported separatists in Ukraine's southeastern regions, and another 1,667 servicemen were listed as missing.

The Kyiv authorities are holding only 223 members of illegal armed groups operating in the east of the country, Vovk said.