Gaza's largest hospital has become a "death zone," the World Health Organisation said on Sunday, announcing plans to evacuate the facility, as Israel's army said it was expanding operations to destroy Hamas.

The assessment came after a visit by WHO and other UN officials to the hospital, which Israeli troops raided earlier this week.

Israel has vowed to destroy Hamas in response to the October 7 attacks, which Israeli officials say killed about 1,200 people, most of them civilians, and saw about 240 people taken hostage.

The army's relentless air and ground campaign has since killed 12,300 people, more than 5,000 of them children, according to the Hamas government which has ruled Gaza since 2007.

The UN says some 1.6 million people have been displaced inside the Gaza Strip by six weeks of fighting, and Israel said on Saturday its military was now "expanding its operational activities in additional neighbourhoods in the area of the Gaza Strip."

Gaza's largest hospital, Al-Shifa, has been a key focus in recent days, with Israeli forces alleging Hamas uses it as a command centre – a claim denied by the group and medical staff.

On Sunday, the WHO said a UN assessment team reached the hospital and found a "death zone", with a mass grave at the entrance and nearly 300 patients left inside with 25 health workers.

"WHO and partners are urgently developing plans for the immediate evacuation of the remaining patients, staff and their families," the body said, warning however that nearby facilities were already overstretched.

It urged an immediate ceasefire given the "extreme suffering of the people of Gaza."

On Saturday, hundreds of people fled the hospital on foot on orders from the Israeli army, according to the facility's director.

Columns of sick and injured – some of them amputees – were seen making their way towards the seafront along with displaced people, doctors, and nurses, as loud explosions were heard around the complex. (AFP)