Half of Gazans are experiencing "catastrophic" hunger, with famine projected to hit the territory's north by May without urgent intervention, a UN-backed food security assessment said Monday.

"To have 50 percent of an entire population in catastrophic, near famine levels, is unprecedented," Beth Bechdol, the deputy director-general of the UN's Food and Agriculture Organization (FAO), told AFP.

It amounts to around 1.1 million people experiencing "catastrophic food insecurity" due to the Israel-Hamas war, the Integrated Food Security Phase Classification (IPC) partnership said in its report.

The IPC hunger monitoring system, carried out by the United Nations and aid agencies, rates hunger levels from one to five and is used by the UN or governments in deciding whether or not to declare a famine.

The situation in the north of Gaza is particularly dire, and aid agencies have reported difficulty accessing the area to distribute food and other aid.

"Famine is imminent in the northern governorates and projected to occur anytime between mid-March and May 2024," the IPC report said.

"All evidence points towards a major acceleration of deaths and malnutrition. Waiting for a retrospective famine classification before acting is indefensible," it said.

Donors have turned to deliveries by air or sea, but air and sea missions are no alternative to land deliveries, UN agencies say.

Famine will occur under "the most likely scenario", based on assumptions that the conflict will escalate -- including a ground offensive in Rafah -- and hostilities continue to impede humanitarian aid delivery, the IPC report said.