Famine spread to a sixth southern Somali region and will likely extend further in the coming four months with 750,000 people at risk of death, the United Nations said Monday.

"Tens of thousands of people have already died, over half of whom are children," according to a statement from the UN's food security analysis team for Somalia, which said the Bay region was now declared a famine zone.

"In total, four million people are in crisis in Somalia, with 750,000 people at risk of death in the coming four months in the absence of adequate response."

The Bay area, which includes the major town of Baidoa, is a stronghold of hardline Islamist Shebab insurgents who have imposed severe restrictions on aid into the areas they control.

"Assuming current levels of response continue, famine is expected to spread further over the coming four months," the statement from UN Somalia Food Security Nutrition Analysis Unit (FSNAU) said.

Famine was first declared in the southern Bakool and Lower Shabelle regions of southern Somalia in July.

It later spread to three further areas, including into the Somali capital Mogadishu and the Afgoye corridor, the world's largest camp for displaced people.

Al-Qaeda affiliated Shebab fighters pulled out of positions in Mogadishu last month, but still control much of southern Somalia, the worst-hit region by famine and the extreme drought.

Famine implies that at least 20 percent of households face extreme food shortages, acute malnutrition in over 30 percent of people, and two deaths per 10,000 people every day, according to UN definition.

Several areas are at severe risk of tipping over into famine conditions, it added.

"An additional 50,000 people in cropping areas of Gedo and Juba and pastoral areas of Bakool face famine-level food deficits," the statement read.

Some 12.4 million people in the Horn of Africa, including parts of Ethiopia, Djibouti, Kenya and Uganda, are affected by the worst drought in decades in the region and are in need of humanitarian assistance, according to the UN.

Meanwhile in Mogadishu, Somali leaders are holding a rare three-day conference that began Sunday in an effort to set up plans for a new government.

The current government is one of the more than a dozen attempts to form a central authority in Somalia since it plunged into war with the 1991 ouster of president Mohamed Siad Barre.

The Somali government was formed in neighbouring Kenya in 2004 with a five-year mandate to reconcile the conflict-shattered country, write a new constitution and hold elections.

But constant political bickering and a near-endless civil war have thwarted progress and the government's mandate renewed twice since. It has also seen two presidents and five prime ministers.