Gaza's health ministry said Israeli forces detained hundreds of staff, patients and displaced people during a raid on Friday on the last functioning hospital in the territory's embattled north.
"Israeli forces have stormed and are present inside Kamal Adwan Hospital" in the city of Jabalia, the ministry said in a statement.
"They are detaining hundreds of patients, medical staff and some displaced individuals from neighbouring areas who sought refuge in the hospital from continuous bombardment."
World Health Organization chief Tedros Ghebreyesus said that contact with the hospital had been lost since Friday morning.
"Since this morning's reports of a raid on Kamal Adwan Hospital in northern Gaza, we have lost touch with the personnel there," Ghebreyesus said on X.
"This development is deeply disturbing, given the number of patients being served and people sheltering there."
The Israeli army confirmed that its troops were operating in the hospital area, accompanied by agents of the Shin Bet domestic security service.
Army and Israeli Security Agency forces "are operating in the area of the Kamal Adwan Hospital in Jabalia, based on intelligence information regarding the presence of terrorists and terrorist infrastructure", it said in a statement.
Israeli forces had surrounded the hospital in Jabalia refugee camp before entering the premises, Gaza's civil defence agency said.
"More than 150 patients and staff, including medical and nursing teams, are besieged by the Israeli army inside Kamal Adwan Hospital," agency spokesman Mahmud Bassal said.
COGAT -- the Israeli defence ministry body responsible for civil affairs in the Palestinian territories -- said Friday it had allowed the transfer of 23 patients out of the hospital the previous night in Palestinian ambulances and UN vehicles.
Kamal Adwan is the last functioning hospital in north Gaza. It has been struggling with shortages of medicines and medical equipment since the start of war, which have been aggravated by the launch of a major Israeli operation in north Gaza earlier this month.
"There has been no supply or provision of food, medicine, or essential medical supplies needed to save the lives of the injured and sick in the hospital," the health ministry said, calling the situation "catastrophic in every sense of the word".
COGAT said it had allowed the transfer of one fuel truck, "180 blood units and a truckload of medical equipment" donated by UN agencies.
Tedros said the WHO and partner agencies had reached the hospital late Wednesday and managed to transfer 23 patients and 26 caregivers to Gaza's Al-Shifa Hospital.
"Kamal Adwan Hospital has been overflowing with close to 200 patients - a constant stream of horrific trauma cases. It is also full of hundreds of people seeking shelter," he said.
Hamas called the storming of Kamal Adwan "a war crime and a flagrant violation of international laws."
Israel launched a major operation in north Gaza on October 6 that has killed 770 people, according to civil defence agency figures.
"Since the start of operational activity in Jabalia, approximately 45,000 Palestinian civilians have evacuated, and IDF (Israeli army) troops have eliminated hundreds of terrorists", the Israeli military said.