Poor nutrition, solitary confinement — details of how the hostages seized when Hamas attacked southern Israel have been treated in captivity are starting to emerge from under a veil of secrecy.

Palestinian militant groups have released more than 50 Israeli women and children since a Qatar- and Egypt-mediated deal took effect on Friday, along with several other foreign nationals, among them at least 17 Thais.

More than 160 other hostages taken in the Hamas attacks on October 7 are still held in the Gaza Strip.

None of the hostages released under the truce have so far given any direct accounts of the conditions in which they were held.

Hospitals say they have been instructed to refrain from disclosing details about the conditions, lest those reports harm those still held captive.

But some details are slowly surfacing from medical professionals treating them, while relatives — often demanding that the Israeli government ensure all the hostages are freed — are offering more dramatic accounts of mistreatment and hardship.

Ronit Zaidenstein, head of the medical team at Shamir Medical Center where 17 released Thai nationals were treated, said they had been fed "very unnutritious food" in captivity.

"The people who came to us lost a significant amount of their body weight in such a short time –- 10 percent or more."

In an interview that has since been taken offline, Margarita Mashavi, a doctor at Wolfson Medical Centre — one of the main facilities caring for freed hostages — said those she spoke to described being kept several stories underground.

"They didn't give them light. They gave it to them for only two hours," she was quoted as saying by the Ynet news site on Monday.

'Very withdrawn'

The patients told her meals consisted of "rice, canned hummus and fava beans, and sometimes salted cheese with pita, but not more than that. No fruit, no vegetables, no eggs," she said.

Food supplies have run short in the Palestinian territory during the war, and the World Food Programme has warned of "widespread hunger".

"Even when they asked for a pencil or pen to write in order to pass the time, the Hamas men didn't allow it because they were afraid they would transmit information in writing, so they were without television or reading, and therefore passed the time only in conversation with one another," Mashavi said.

She referred an AFP interview request to her employers, who declined.

Esther Yaeli, grandmother of 12-year-old French-Israeli boy Eitan Yahalomi, who was released on Monday, told the Walla news website he was held in solitary confinement for 16 days.

"The days that he was alone were horrible," she said. "Now Eitan appears very withdrawn."

"The noises of the bombs hurt him, his ears hurt for a very long time," Yaeli said he told her.

The returned hostages have arrived after nightfall, and are immediately assessed to determine if any need urgent medical care.

Two of the freed hostages have been hospitalised after their release, including 84-year-old Elma Avraham, who was treated in intensive care but whose condition doctors said on Tuesday had improved.

Hagar Mizrahi, the head of the Israeli health ministry's operations for returning hostages, told AFP that they had been held in "horrible conditions" and that "the medical consequences are pretty clear".

She declined to elaborate, citing patient privacy concerns.

"Some of the things that I've heard in recent days are heart-wrenching," she added, offering no specifics. "They're simply outrageous in every way."