Israeli forces are intensifying their attacks on areas around the Al-Shifa Hospital, the largest medical facility in the Gaza Strip.
The Israeli military claims that a key stronghold of the Islamic group Hamas is located under the hospital.
It has issued evacuation warnings to medical staff, patients and residents at Al-Shifa and two other hospitals in Gaza City.
The military also announced on Sunday that its troops had started full-scale fighting against Hamas at the al-Shati refugee camp in Gaza City.
The World Health Organization said in a social media post on Sunday that it "has lost communication with its contacts" in the hospital. The WHO said that before losing contact, it had received a report that "the hospital was surrounded by tanks."
It also said, "There are reports that some people who fled the hospital have been shot at, wounded, and even killed."
A spokesperson for the health authorities in Gaza said the Al-Shifa hospital had been forced to halt medical activity as it was running short of fuel, adding that the lives of babies in incubators are at risk.
But Israeli military spokesperson Rear Adm. Daniel Hagari told reporters on Saturday that its forces had not attacked the hospital.
He also said the military is speaking directly with hospital staff. He said Israeli troops will assist on Sunday in moving babies in the facility's pediatric department "to a safer hospital."
The spokesperson repeated Israel's criticism of Hamas, stressing that 239 men, women, children, elderly and even babies are being held hostage by the group.
The Israeli military said it will halt military activity in part of northern Gaza for four hours from 10 a.m. local time on Sunday, following a similar move the previous day.