The Israeli military says its special forces are still inside the Nasser hospital in Gaza as fears grow for patients at the site.
Israel launched what it described as a "precise and limited mission" there on Thursday. The military says it has caught "dozens of terror suspects".
Hamas dismissed that claim as "lies". The Hamas-run health ministry said five people died after generators failed.
The World Health Organization (WHO) said the facility urgently needed fuel.
It said the fuel was vital to "ensure the continuation of the provision of life-saving services".
Tarik Jasarevic, a WHO spokesperson, said there were now reports that the orthopaedic unit at the hospital, in the city of Khan Younis, had been damaged.
"That obviously reduces the ability to provide the urgent medical care," he said, adding there were still "critically injured and sick patients" at the hospital.
"More degradation to the hospital means more lives being lost."
Nasser is the main hospital in southern Gaza, and is one of the few still functioning. It has been the scene of intense fighting between the IDF and Hamas for days.
