Famine is set to spread to two new southern Somali regions with millions of people reeling from extreme drought, the top UN humanitarian aid official for the war-torn nation warned on Wednesday.

"We are anticipating that two more areas of southern Somalia will be included," Mark Bowden, the UN Humanitarian Coordinator for Somalia, told AFP.

Middle and Lower Juba are expected to be added to five other regions already declared by the UN to be in famine, including parts of the war-ravaged capital Mogadishu, he said.

"These areas were anticipated, and we are waiting only for the statistical analysis to confirm," Bowden added.

The UN has described Somalia, where a civil war has been going on since 1991, as facing the most severe humanitarian crisis in the world.

Last month, the UN declared famine in the southern Bakool and Lower Shabelle regions of southern Somalia. It later spread to three further areas, including the Afgoye corridor, the world's largest camp for displaced people.

Famine implies that at least 20 per cent 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.

"It (the famine declaration) stimulated a rapid increase in international funding and support," said Bowden, adding that up to $280 million of funds pledged had already been received.

Of the $1 billion requested for Somalia, 57 percent has been met, the UN official said, including $350 million promised last week by the Organisation of Islamic Cooperation.

African leaders will also meet Thursday to pledge funds to tackle the crisis.

"The initiative is very welcome," Bowden added. "It provides encouragement for traditional donors to have the involvement of Africans helping Africans in this crisis."

Al-Qaeda affiliated Shebab gunmen pulled out of positions in Mogadishu earlier this month, but continue to restrict aid into areas they control in famine-hit southern Somalia regions.

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.