Two major hospitals in northern Gaza closed to new patients on Sunday, staff said.
The Hamas-run Gaza Health Ministry said the Al-Shifa Hospital's last generator ran out on Saturday as fighting intensified around the compound into Sunday.
"The situation in Al-Shifa is catastrophic," doctor Mohammad Zaqut told the AFP news agency.
Israeli military spokesperson Richard Hecht said on Sunday the army had left 300 liters (634 pints) of fuel near Al-Shifa Hospital overnight to power an emergency generator for newborn incubators. He said the fuel was not taken.
The hospital's director, Mohammad Abu Salmiya, told journalists that 300 liters was enough to power its generators for "no more than a quarter of an hour."
Doctors Without Borders (MSF) said that they had lost contact with their staff at Al-Shifa — the largest medical facility in the Gaza Strip.
Meanwhile, the Palestinian Red Crescent said the Al-Quds Hospital is "no longer operational" due to a lack of generator fuel.
"The hospital has been left to fend for itself under ongoing Israeli bombardment, posing severe risks to the medical staff, patients and displaced civilians," it added. Israel has denied bombing hospitals.
Alyona Synenko, a spokesperson for the International Red Cross (ICRC) in Jerusalem, told DW that the conditions in the south of Gaza are "supposed to be better" but that the medical facilities there were still "facing enormous challenges."
"Every day they are receiving new patients, patients with severe wounds, a lot of burns, and they're running out of a lot of essential supplies, like gauze, like anesthetics," she said, adding that many young children were among them.