Three new areas of Somalia have been classified as having been hit by famine, the UN says.
It declared a famine in two large southern regions of the war-torn country in July.
"Famine is expected to spread across all regions of the south in the coming four to six weeks," said the UN Food Security and Nutrition Analysis Unit.
More than 11 million people have been affected by the worst drought in 60 years in the Horn of Africa.
The UN's Food Security and Nutrition Analysis Unit (FSNAU) said that famine was "likely to persist until at least December 2011".
Evidence from malnutrition and mortality rates shows that famine thresholds have been surpassed in two rural districts of the Middle Shabelle region - Balcad and Cadale - as well as the parts in and around the capital, Mogadishu, where there are camps for displaced people.
These three areas join the Bakool and the Lower Shabelle region, where famine was declared on 20 July 2011.
"A humanitarian emergency persists across all other regions of southern Somalia, and tens of thousands of excess deaths have already occurred," the UN unit said in a joint statement with the US-based Famine Early Warning Systems Network (Fewsnet).
Getting aid into Somalia has been difficult because the al-Qaeda-linked al-Shabab group - which controls much of the south and central regions and parts of the capital - has banned some aid agencies from their territory.
On Thursday, US Secretary of State Hillary Clinton called on the militants to let the aid through.
"Al-Shabab are preventing assistance to the most vulnerable populations in Somalia," she said.
"I call on al-Shabab to allow assistance to be delivered in an absolutely unfettered way throughout the area... so that as many lives as possible can be saved."
Some 3.2 million people in Somalia are in need of immediate life-saving assistance - almost half the population, the UN says.
According to the FSNAU and Fewsnet, the situation has been compounded by the rise in prices of food staples in Somalia - they have more than doubled since 2010, and in some areas have tripled.
"Across all livelihoods, poor households (30% of the population) are unable to meet basic food needs and have almost no ability to cope with these food deficits," their statement said.
It is the first time in 19 years that the country - which has been without a central government since 1991 - has experienced famine.