"Twenty-three hospitals have been ordered to evacuate in Gaza City and north Gaza, and forced evacuation in these circumstances would put the lives of hundreds of patients in a life threatening situation," WHO chief Tedros Adhanom Ghebreyesus said.

At a press conference in Geneva, Tedros also reiterated the call for a humanitarian pause in the Israel-Hamas war to help the thousands injured, as well as the chronically sick.

More than 10,000 people have been killed, including more than 8,500 in Gaza and 1,400 in Israel, he said, mainly women and children on both sides.

There are also more than 21,000 people injured, as well as 1.4 million displaced in Gaza, he said, as well as many more with long-term conditions who needed care.

"We're running out of words to describe the horror unfolding in Gaza," said Tedros.

The WHO said 46 critically injured medical evacuees had left Gaza for care in Egypt since the Rafah crossing opened on Wednesday, and it had also provided 54 metric tonnes of medical aid to the territory, but nowhere near what was needed.

The situation in Gaza was "indescribable", Tedros added, with hospitals crammed, morgues overflowing, and doctors performing surgery without anaesthesia, as families sought shelter and toilets overflowed, risking the spread of disease.

He also reiterated calls for Hamas to release Israeli hostages, many of whom need medical attention.

Emergencies director Dr Mike Ryan said the U.N. agency was struggling to help as it could not guarantee the safety of staff.

"We have never found it as difficult to establish the basic rules of engagement that would allow us to act in a proper humanitarian fashion," he said.

He said any international staff or field hospitals entering Gaza would need safety guarantees, and to run alongside the existing system to be effective.

"We will not be instrumentalized in this. We will not become a party to this... Our job is to save lives. That is our only job," he said.