The Israeli military has raided Gaza's main hospital in what it describes as a "targeted operation against Hamas".
An eyewitness at Al-Shifa hospital told the BBC that troops moved in overnight and were interrogating people.
On Wednesday, Israel said it found Hamas' "operational centre" at the hospital, sharing images of what it said were Hamas weapons and equipment.
Hamas denies operating there and the BBC cannot independently verify claims by either side.
UN humanitarian chief Martin Griffiths said he was "appalled" by the Israeli raid, while the World Health Organization (WHO) said it was "extremely worried" for patients and staff, with whom it had lost contact.
On Wednesday evening, the IDF said troops found "an operational command centre, weapons, and technological assets" belonging to Hamas inside the MRI building.
It said it was "continuing to operate in the hospital complex", sharing images and videos showing what it said were Hamas weapons.
In a seven-minute video, IDF spokesperson Jonathan Conricus pointed to security cameras that he said had been covered over, and weapons that he said were AK47 rifles hidden behind MRI scanners.