Ukraine and Russia have traded accusations over the missile strike or explosion in the front-line town of Olenivka, in eastern Donetsk, that killed prisoners held by Moscow-backed separatists.
International Committee of the Red Cross Director-General Robert Mardini told reporters in Kyiv the group was engaged in intense negotiations with Russian authorities but had not been granted access to the POWs and lacked the security guarantees required for a visit.
The Red Cross registered 1800 people taken from the besieged Azovstal steel works in the Ukrainian port of Mariupol, with the understanding it would be allowed to visit them, but that has not been possible, he told a news conference.
Olenivka is about 90km north of Mariupol.
Mardini said the Red Cross had visited hundreds of POWs on both sides of the war, now in its seventh month, but there were thousands more it had not been able to see.
"We are negotiating every day to have full access to all prisoners of war," Mardini said.
"It is clearly an absolute obligation (of) the parties to give the ICRC access to all prisoners of war."
He declined to provide details about conditions in prisons the Red Cross had visited.
Mardini said the group was discussing the lack of access to prisoners at the Olenivka site with officials at "every level" of the Russian government and was engaged in a "very constructive" dialogue with the Ukrainian government about what he called an unsatisfactory situation.
Mardini said the Red Cross had facilitated the transport of more than 1000 letters from Ukrainian troops being held in Russia to Ukraine, where they will be delivered to their families.
He said the aid group had provided more than 3000 families with news of their loved ones since the start of the war, which Russia calls a "special military operation".
