The United Nations has formally declared a famine in two areas of southern Somalia as it suffers the worst drought in more than half a century.
The UN says the humanitarian situation in southern Bakool and Lower Shabelle has deteriorated rapidly, despite assistance efforts.
"If we don't act now, famine will spread to all eight regions of southern Somalia within two months, due to poor harvests and infectious disease outbreaks," UN official Mark Bowden said.
"If we are not able to intervene immediately, tens of thousands more Somalis may die."
The UN says an estimated 3.7 million people - or nearly half of the war-torn country's population - are facing a food crisis.
More than 166,000 Somalis are estimated to have fled their country to neighbouring Kenya or Ethiopia.
United States officials say they will allow aid to be sent to famine-affected areas of Somalia controlled by an Islamist militia, provided none of it is used to benefit the militants.
A representative of the American agency USAID said it would need the UN's assurance that the aid would not be taxed by al Shabaab.
A US State Department official, Reuben Brigety, says al-Shabaab was largely responsible for the dire humanitarian situation in the regions it controls.
Across East Africa, an estimated 10 million people have been affected by the worst drought in more than half a century. Somalia is the worst affected nation but parts of Kenya, Uganda, Ethiopia and Djibouti are also hit.
According to a UN definition, famine implies that at least 20% of households face extreme food shortages, more than 30% of people suffer acute malnutrition and two deaths per 10,000 people occur every day.
Malnutrition rates in Somalia are the highest in the world, with peaks of 50% in certain areas of southern Somalia, Mr Bowden said.
On Tuesday, the UN refugee agency said death rates among refugees arriving in Ethiopia's Dolo Ado area reached 7.4 per 10,000 in June, 15 times more than the baseline rate in sub-Sahara Africa.
The Food and Agricultural Organisation appealed on Wednesday for $US120 million for the victims in the Horn of Africa.