The World Health Organization said Friday that a mass evacuation of the northern Gaza Strip would be "disastrous" for hospital patients, with hospitals in the south already at full capacity.

Israel has given Palestinians 24 hours to leave the northern Gaza Strip ahead of an expected ground offensive in retaliation against Hamas for the deadliest attack in Israeli history.

"The Palestinian Ministry of Health has informed WHO that it is impossible to evacuate vulnerable hospital patients without endangering their lives," the UN health agency said in a statement.

"Vulnerable patients include those who are critically injured or dependent on life support. Moving them amid hostilities puts their lives at immediate risk."

The WHO joined other UN bodies in calling for Israel to rescind the evacuation order.

"A mass evacuation would be disastrous — for patients, health workers and other civilians left behind or caught in the mass movement," it said.

The two major hospitals in the north of the Gaza Strip have already greatly exceeded their combined 760-bed capacity, "with severe overcrowding", the WHO said.

"Of the thousands of patients with injuries and other conditions receiving care in hospitals, there are hundreds that are severely wounded and over 100 who require critical care. These are the sickest of the sick," it said.

Many thousands more, also with wounds or other health needs, cannot access any kind of care, the WHO noted.

Furthermore, the four health ministry hospitals in the southern Gaza Strip are already at or beyond capacity, and lack the critical care capacity and supplies needed to treat additional patients, said the WHO.

WHO spokesman Tarik Jasarevic earlier told reporters in Geneva that the Gaza Strip's health system was now at "breaking point", with six of the seven main hospitals only partially functioning.

WHO calls for "an end to hostilities and violence in the Gaza Strip, where unimaginable human suffering is unfolding" the spokesman said.

Time 'running out' 

Concern is rising over the worsening humanitarian crisis in the long-blockaded Palestinian enclave, where Israel has now imposed a total siege, cutting off water, food and energy supplies.

With dwindling supplies of food, water and health services and without adequate shelter, people will be "at heightened risk of disease", said the WHO.

The UN agency's pre-positioned supplies in Gaza have mostly been consumed.

The WHO is calling for an immediate humanitarian corridor to be established into the Gaza Strip.

Supplies which WHO had pre-positioned in Gaza have mostly been consumed.

The organisation has prepared medical supplies for more than 300,000 patients with a range of wounds and diseases at its logistics hub in Dubai. It stands ready to deliver them to Arish in Egypt — close to the southern Rafah crossing into Gaza.

"Time is running out to prevent a humanitarian catastrophe if fuel, water, food and life-saving health and humanitarian supplies cannot be urgently delivered to the Gaza Strip amidst the complete blockade," said Jasarevic.