An estimated 10 million people have been affected by the drought in east Africa. The United Nations has declared a famine in two areas of southern Somalia as it suffers the worst drought in more than half a century.
The UN said the humanitarian situation in southern Bakool and Lower Shabelle had deteriorated rapidly.
It is the first time that the country has seen famine in 19 years.
Meanwhile, the UN and US have said aid agencies need further safety guarantees from armed groups in Somalia to allow staff to reach those in need.
Al-Shabab, an al-Qaeda-affiliated group which controls large swathes of south and central Somalia, had imposed a ban on foreign aid agencies in its territories in 2009, but has recently allowed limited access.
An estimated 10m people have been affected in east Africa by the worst drought in more than half a century. More than 166,000 desperate Somalis are estimated to have fled their country to neighbouring Kenya or Ethiopia.
'Rarely used' Drought, conflict and poverty have now combined to produce the necessary conditions for famine. Those conditions include more than 30% of children being acutely malnourished, and four children out of every 10,000 dying daily.
Definition of Famine
- More than 30% of children must be suffering from acute malnutrition
- Two adults or four children must be dying of hunger each day for every group of 10,000 people
- The population must have access to far below 2,100 kilocalories of food per day
It said that the ongoing conflict had made it extremely difficult for agencies to access communities in the south, which are controlled by al-Shabab.
"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," the head of the agency Mark Bowden warned.
The BBC's Africa correspondent Andrew Harding says the emotive word "famine" is used rarely and carefully by humanitarian organisations, and it is the first time since 1992 that the word has been applied to a situation in Somalia.
The UK Secretary of State for International Development, Andrew Mitchell, said the response by many European and developed countries to the crisis in the Horn of Africa had been "derisory and dangerously inadequate".
"The fact that a famine has been declared shows just how grave the situation has become. It is time for the world to help," he said.
Meanwhile, the UN is calling for unhindered access to affected areas, saying that the security situation is hampering humanitarian efforts.
Adrian Edwards, a spokesman for the UN refugee agency, told reporters that the situation for aid workers in Somalia is "not what we want it to be".
"We do have a very minimal presence, and we have regular visits into the country, but we need significantly better access than we have at the moment to address an emergency of this scale," he said, speaking from Geneva.
The UN World Food Programme, which is trying to feed 1.5 million people, estimates that as many as one million people are in areas it cannot currently access.
"Once we have the assurances of security and the ability to have full access to deliver and distribute and monitor, then we will be prepared to go back in," Emilia Casella, a spokeswoman for the WFP, told the Associated Press news agency.
Johnnie Carson, the US assistant secretary of state for African affairs, says the US was assessing if they were seeing "real change" from al-Shabab, or whether the group planned to impose some kind of "taxation" on aid deliveries.
"Al-Shabab's activities have clearly made the current situation much worse," Mr Carson said.
"We call on all of those in south-central Somalia who have it within their authority to allow refugee groups and organisations to operate there to do so," he said.