Around 80 unidentified Palestinian bodies have been buried in a mass grave in Rafah, southern Gaza.
Sources in the Hamas-run health ministry told AFP news agency they had been transported "from morgues and graves" to Israel for authorities to check there were no hostages among them.
They were later returned through the Kerem Shalom crossing in southern Gaza.
Israel's army says it is checking the information.
Israel says 129 out of 240 people are still unaccounted for after they were abducted and taken to Gaza on 7 October. The attacks on that day, led by Hamas, killed 1,200 people.
Over 20,600 Palestinians have been killed in Israeli bombardments since then, according to Gaza's health ministry.