The United Nations says famine will spread across Somalia unless millions of dollars worth of emergency aid is distributed in the region.
The United States has said it will allow aid to be distributed in two regions where famine has already been declared.
It had previously been reluctant because the area was controlled by the militant islamist group al Shabaab, which is regarded as a terrorist organisation.
The UN’s humanitarian coordinator for Somalia Mark Bowden told a news conference in Nairobi: ‘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’.
Years of drought, that have also affected Kenya and Ethiopia, have hit harvests and conflict has made it extremely difficult for agencies to operate and access communities in the south of the country.
Much of southern and central Somalia is controlled by Islamist militants linked to al Qaeda who imposed a ban on food aid in 2010, which the UN and the US said had exacerbated the crisis. The rebels lifted the ban this month.
In the worst-affected areas, half the children are malnourished.
The UN yesterday declared famine in Somalia's southern Bakool and Lower Shabelle regions. The agency is poised to launch its largest ever relief effort, Mr Bowden said.
US Secretary of State Hillary Clinton pledged an extra $28m (€19.6m) in aid but international charities have criticised the slow response of many Western governments to funding an estimated $800m (€562.2m) shortfall.
The UN was exploring ways of providing ‘cash relief’ while it finds ways of getting larger volumes of food aid into southern Somalia, Bowden said. The global body has appealed for $300m (€210.8m) over the next two months for Somalia.
Nearly 135,000 Somalis have fled since January, mainly to Kenya and Ethiopia. An average of 1,700 and 1,300 Somalis are arriving in Ethiopia and Kenya respectively each day.
The UN defines famine as at least 20% of households facing extreme food shortages, a crude mortality rate of more than 2 people per 10,000 per day and malnutrition rates of above 30%.
