The International Committee of the Red Cross (ICRC) has not yet received permission to visit the site of Friday's attack that killed dozens of Ukrainian prisoners of war, and has condemned the incident at the Olenivka facility in eastern Ukraine.
"Families must receive urgent news of and answers on what happened to their loved ones," the ICRC said in a statement on Sunday.
"The parties must do everything in their power, including through impartial investigations, to help determine the facts behind the attack and bring clarity to this issue.
"However, it is not the role or mandate of the ICRC to carry out public investigations into alleged war crimes."
Russia on Sunday invited United Nations and Red Cross experts to probe the deaths of dozens of prisoners held by Moscow-backed separatists.
Ukraine and Russia have traded accusations over the missile strike or explosion early on Friday that appeared to have killed dozens of Ukrainian prisoners in the front-line town of Olenivka.
The Russian Defence Ministry on Saturday published a list of 50 Ukrainian prisoners of war killed and 73 wounded in what it said was a Ukrainian military strike with a US-made High Mobility Artillery Rocket System (HIMARS).
Ukraine's armed forces have denied responsibility, saying Russian artillery attacked the prison to hide mistreatment there.