The Israeli military has entered Gaza's main hospital in what it describes as a "targeted operation against Hamas".
An eyewitness at Al-Shifa hospital told the BBC troops moved in overnight and were interrogating people.
Israel has long accused Hamas of having a command centre under Al-Shifa and the US has said its intelligence backs this up, but Hamas has denied it.
UN humanitarian chief Martin Griffiths said he was "appalled" by the raid and that "hospitals are not battlegrounds".
Meanwhile, the World Health Organization said it was "extremely worried" for patients and staff, with whom it had lost contact.
The BBC has been speaking to a journalist and a doctor inside the hospital to try to find out what is happening there, while the Israel Defense Forces (IDF) have also been providing updates. We cannot independently confirm any of the accounts.
Eyewitness account from inside hospital
Khader, a journalist inside Al-Shifa, told the BBC's Rushdi Abu Alouf that Israeli troops were in "complete control" of the hospital and no shooting was taking place.
He said six tanks and about 100 commandos entered the complex during the night, with soldiers shouting "Don't move!" inside the main emergency department.
The Israeli forces then went room to room, floor by floor, questioning both staff and patients and were accompanied by medics and Arabic speakers, he added.
Over loudspeakers, the IDF asked all men between the ages of 16 and 40 to leave the hospital buildings, except the surgical and emergency departments, and go to the hospital courtyard.
Soldiers fired into the air to force those remaining inside to come out, Khader said.
He also said troops installed a scanning and sensor device and asked the men to pass through it.
Muhammad Zaqout, the Hamas-run Gaza health ministry's director of hospitals, has also given an account of how the raid played out.
In an interview with Al Jazeera, he said "not a single bullet" had been fired - because "there are no resistors or detainees" inside.
Israel describes 'targeted operation' against Hamas
The IDF announced in a statement early on Wednesday that its forces were in the midst of a "precise and targeted operation against Hamas in a specified area in [Al-]Shifa Hospital".
It said the raid was "based on intelligence information and an operational necessity", and it called for the surrender of "all Hamas terrorists present in the hospital".
Later on Wednesday, the IDF said that before entering Al-Shifa its troops "encountered explosive devices and terrorist cells, and an engagement began in which terrorists were killed".
A senior IDF official, who declined to be named, told reporters that Israel had found weapons and "terror infrastructure" but they did not immediately provide evidence.
No fighting had taken place within the hospital and there was no friction between soldiers and people there, the official said.
Israel's Army Radio also reported that troops had not yet found any sign of any of the 240 hostages taken by Hamas gunmen during the 7 October attack on southern Israel, when 1200 other people were killed.
The raid on Al-Shifa came shortly after the US publicly backed - for the first time - Israeli claims that Hamas had infrastructure underneath the hospital.
White House National Security spokesperson John Kirby said the US had its own intelligence, which had come from a variety of sources, suggesting Hamas used hospitals in the Gaza Strip and tunnels underneath them to conceal military operations and hold hostages.
"Hamas and Palestinian Islamic Jihad members operate a command and control node from Al-Shifa in Gaza City," he said, adding that it showed "how challenging [the Israeli] military operation is, because Hamas has deeply embedded itself within the civilian population".
However Dr Ahmed Mokhallalati, a plastic surgeon at Al-Shifa reached by the BBC, insisted that there were only civilians in the hospital and said he had never seen a single gun inside the hospital, or any Hamas presence.
He said there were tunnels under every building in Gaza, including Al-Shifa hospital.
Worsening humanitarian situation
Intense fighting has been reported around Al-Shifa for days, trapping patients, staff and displaced people who had sought shelter there.
Dr Mokhallalati told the BBC on Wednesday that the hospital was without power, oxygen and water.
On Tuesday, essential surgeries had been carried out without proper anaesthesia, with patients "screaming in pain", he said. No surgeries could be carried out on Wednesday.
Doctors were unable to help one patient with burns on Tuesday due to lack of equipment including ventilators and had to just "let him die", Dr Mokhallalati added.
He also said that six premature babies had died in recent days and that he feared more would die due to lack of oxygen and lack of power.
The IDF said its troops were providing incubators, baby food and medical supplies to the hospital.
"Why can't they be evacuated" Dr Mokhallalati said of the babies. "In Afghanistan, they evacuated the cats and dogs."
"Where is the ICRC (International Committee of the Red Cross)?" he added. "Where are the British and American governments? Is everyone just waiting for us all to die here and then say we were 'good people'?"
Although Israel previously said it was ready to allow staff and patients to evacuate, Palestinians have said Israeli forces opened fire at them and that it was too dangerous to move vulnerable patients.
Witnesses have described dire conditions inside the hospital, with families with scant food or water living in corridors and smell of decomposing bodies in the air.
US 'does not want firefight in hospital'
Before the raid, Israel had said it was not targeting hospitals directly but acknowledged "clashes" around Al-Shifa and other facilities had occurred in recent days.
Asked to comment on the operation, the White House said civilians and patients had to be protected and it did not want to "see a firefight in a hospital where innocent people... are trying to get medical care they deserve".
Griffiths, the UN's under secretary general for humanitarian affairs, wrote on X, formerly known as Twitter: "The protection of newborns, patients, medical staff and all civilians must override all other concerns."
In a separate video statement Griffiths said he understood that Israel wanted to find Hamas leaders but his agency's main concern was for the welfare of the people at Al-Shifa.
"Hamas must not, should not, use a place like a hospital as a shield for their presence," he said.
"I understand the Israelis' concern for trying to find the leadership of Hamas. That's not our problem. Our problem is protecting the people of Gaza from what's being visited upon them," he added.
The ICRC said in a statement that it was "extremely concerned about the impact on sick and wounded people, medical staff, and civilians".
- This story was first published by the BBC