A senior Palestinian doctor died in an Israeli prison after more than four months of detention, two Palestinian prisoner associations said on Thursday, blaming Israel for his death.

The associations said in a joint statement that Adnan Al-Bursh, head of orthopedics at Al Shifa Hospital, Gaza’s largest medical facility, had been detained by Israeli forces while temporarily working at Al-Awada Hospital in north Gaza.

They called his death an “assassination” and said his body remained in Israeli custody.

An Israeli military spokesperson said that the prison service had declared Bursh dead on April 19, saying that he had been detained for national security reasons in Ofer prison. The spokesperson did not comment on the cause of death.

Medical groups, including the World Health Organization, have repeatedly called for a halt to attacks on Gaza healthcare workers, with more than 200 killed so far in the Gaza conflict, according to an estimate from Insecurity Insight, a research group that collects and analyses data on attacks on aid workers around the world.

The Palestinian health ministry said in a statement that Bursh’s death raised to 496 the number of medical sector workers who had been killed by Israel since Oct 7. It added that 1,500 others had been wounded while 309 had been arrested.

Israel accuses Hamas of using hospitals for military purposes and says its operations against them have been justified by the presence of fighters. Hamas and medical staff deny the allegations.

Earlier on Thursday, the Israeli authorities released 64 Palestinians they had detained during their military offensive in Gaza via the Israeli-controlled Kerem Shalom crossing, the Palestinian borders and crossings agency said.

One of them was the body of another man who had died in detention, the prisoners’ associations said.

Another freed detainee arrived in critical condition and was moved into hospital upon arrival, the crossings agency added.

Dozens of Palestinians who had been freed by Israel in past months including some staff of a U.N. agency have reported ill-treatment during detention, including torture and deprivation of food and sleep.

The two new deaths bring the toll of Gazans who died in Israeli custody to at least 18 since the start of the war, the prisoners associations said, urging Israeli authorities to disclose the number, location and fate of detainees from Gaza.

The U.N. Palestinian Refugee Agency has documented the release of 1,506 people detained by the Israeli authorities through the Kerem Shalom crossing as of April 4 and said the transfer of detainees regularly holds up aid. The 1,506 included 43 children and 84 women, it said.

Israel’s military operation in Gaza was triggered by Hamas’s Oct. 7 attack, which by its tallies killed 1,200 with 253 taken hostage. The subsequent Israeli bombardment has killed more than 33,000 Palestinians, according to Palestinian medics, and displaced the majority of Gaza’s 2.3 million people.