Five Ukrainian servicemen have been killed in the past four days, the military said on Tuesday, underscoring the strains in a cease-fire between government forces and pro-Russian separatists that officials insist is still broadly holding.
The cease-fire, agreed on Friday, is part of a peace plan meant to end a five-month conflict that has killed more than 3,000 people to date.
Russia, accused by Kiev and the West of sending troops into eastern Ukraine and arming the rebels, urged the two sides in the conflict to begin talks soon on the region's final political status. It denies accusations of intervening in the conflict.
Under the terms of the cease-fire, Ukrainian military spokesman Andriy Lysenko said the rebels had so far released 648 prisoners-of-war to the government side. He said the Ukrainian side was working to secure the release of a further 500 POWs.
A rebel leader, Andrei Purgin, told Interfax news agency he expected an exchange of 36 more POWs on Tuesday.
The cease-fire largely held overnight into Tuesday despite sporadic violations, including in rebel-held Donetsk, the region's largest city, where government forces hold the airport. A woman was wounded in Donetsk overnight, officials said.
Lysenko said the death toll among servicemen since the start of the cease-fire now stood at five, with 33 wounded.
At the weekend, one woman was killed and at least four other civilians were wounded when government forces came under heavy shelling near the port of Mariupol on the Sea of Azov.
Both sides say they are observing the cease-fire and blame each other for any violations.
"Russian troops and terrorists are continuing their brazen violations of the conditions of the cease-fire, shooting at the positions of the Ukrainian forces, including with heavy weaponry," defense analyst Dmytro Tymchuk, who has close ties to the Ukrainian military, said in a statement.