The Israeli military has raided Al-Shifa Hospital, the largest medical institution in the Gaza Strip.
The military said on Wednesday that it is conducting what it calls a "precise and targeted operation" against the Islamic group Hamas in a specified area of the hospital.
Satellite broadcaster Al Jazeera quoted a doctor at the hospital as saying that tanks and bulldozers were on the facility's grounds. The doctor also reportedly said fierce gun firing is continuing and explosions can be heard.
CNN reported that according to a local journalist inside the hospital, Israeli tanks had entered the hospital complex.
Israel says Hamas has a command center underneath Al-Shifa.
Speaking to reporters on Tuesday, White House National Security Council spokesperson John Kirby also said Hamas and another group "operate a command-and-control node" from Al-Shifa and use the facility to hold hostages and store weapons.
Hamas has denied the allegations.
According to a UN agency, there are more than 600 patients in the hospital, as well as about 1,500 other residents who sought refuge there. Concern is mounting that attacks on the facility could result in further casualties.
Meanwhile, the White House said on Tuesday that President Joe Biden and Israeli Prime Minister Benjamin Netanyahu have discussed the situation in Gaza over phone.