The biggest hospital in the Gaza Strip and another with children on life support was coming under bombardment on Friday, the World Health Organization said.
Twenty hospitals in Gaza were now out of action entirely, it said.
Asked about the Gaza health ministry's allegation of an Israeli strike on the courtyard of Al Shifa hospital in Gaza City, WHO spokesperson Margaret Harris said: "I haven't got the detail on Al Shifa but we do know they are coming under bombardment."
She said there was also "significant bombardment" on Rantissi hospital, the only hospital providing paediatric services in North Gaza.
Asked to elaborate, she said there was "intense violence" at the Shifa site, quoting colleagues on the ground. She did not attribute blame.
Gaza families have been sheltering at the hospital, the territory's largest, which is inside Gaza City encircled by Israeli troops. Israel says militants have headquarters in the area and has told residents to leave and go south.
The Rantissi hospital has children on life support and receiving dialysis, Harris added, saying it would be impossible to evacuate them safely.
At the same briefing, the U.N. humanitarian office spokesperson Jens Laerke said that there had been some "issues" getting aid into Gaza through the Rafah crossing with Egypt, which it said had been designed for pedestrians, not trucks.
Only 65 trucks carrying food, medicine, hygiene supplies and water, and seven ambulances, crossed from Egypt into Gaza on Wednesday, it said, which is a fraction of pre-conflict levels.
None of that aid can reach northern Gaza, he added.
"We cannot drive to the north at the current point which is of course deeply frustrating because we know there are several hundred thousand people who remain in the north," said Laerke.
"If there is a hell on earth today, it's name is northern Gaza," he said. "It is a life of fear by day and darkness at night and what do you tell your children in such a situation, it's almost unimaginable - that the fire they see in the sky is out to kill them?" he said.
He urged Israel to re-open the Kerem Shalom crossing to allow more aid through.
Israel has started four-hour battle pauses in the northern Gaza strip to enable Palestinians to flee the brunt of its retaliatory attacks on the enclave following a deadly cross-border rampage by Hamas militants on Oct. 7.
However, Laerke criticised the pauses, saying they had not been coordinated with the United Nations.
"Obviously, for this to be done safely for humanitarian purposes it has to be agreed with all parties," he said.
He also voiced concerns about overcrowding in Gazan shelters, as tens of thousands of people stream southwards.
"It can only make the situation of overcrowding in UNRWA facilities – and thank god for them – it can only make it worse," he said, referring to the U.N. Palestinian refugee agency which operates shelters for displaced Gazans.
