Food aid in Sudan is set to run out by the end of March unless new funding is secured, the United Nations has said, raising fears for millions caught up in the world's largest hunger crisis.

"By the end of March, we will have depleted our food stocks in Sudan," Ross Smith, the World Food Programme's Director of Emergency Preparedness and Response, said in a statement on Thursday.

"Without immediate additional funding, millions of people will be left without vital food assistance within weeks," he added.

Smith said the WFP has already "been forced to reduce rations to the absolute minimum for survival".

The warning comes after almost three years of civil war in Sudan, where fighting between the army and paramilitary Rapid Support Forces has left tens of thousands dead and 11 million displaced.

Acute food insecurity

According to the UN, more than 21 million people -- almost half of Sudan's population -- are now facing acute food insecurity.

A UN-backed assessment confirmed last year that famine had taken hold in El Fasher, the capital of North Darfur, which was overrun by the paramilitary forces in October.

Famine has also been confirmed in Kadugli, in neighbouring Kordofan, now a key battleground in the conflict.

In Dilling, around 130 kilometres to the north, the UN says civilians are likely experiencing famine conditions, though security issues and a lack of access have prevented a formal declaration.