Mark Bowden, humanitarian co-ordinator for Somalia, said on Wednesday that famine conditions now existed in the Bakool and Lower Shabelle regions of the country.
He warned: "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.
"We still do not have all the resources for food, clean water, shelter and health services to save the lives of hundreds of thousands of Somalis in desperate need."
He added that the lack of resources is alarming. "Every day of delay in assistance is literally a matter of life or death for children and their families in the famine-affected areas."
UN humanitarian agencies have welcomed the recent statement by al-Shabaab, Islamist insurgents affiliated to al-Qaida, requesting aid in southern Somalia, but said the inability of food agencies to work in the region since early 2010 has prevented the UN from reaching the very hungry - especially children - and has contributed to the current crisis. The Bakool and Lower Shabelle regions are understood to be controlled by al-Shabaab. The UN refugee agency, the UNHCR, said it was seeking further security guarantees from the rebel group that it can deliver greater amounts of assistance in the area to prevent more hungry people from becoming refugees.
The drought in east Africa has left an estimated 11 million people at risk, but Somalia has been the worst hit country as it is already wracked by decades of conflict. The most affected areas of Somalia are in the south, particularly the region of Lower Shabelle, Middle and Lower Juba, Bay, Bakool, Benadir, Gedo and Hiraan, where the UN says an estimated 310,000 now suffer from acute malnutrition.
© Oli Scarff/Getty ImagesA woman and child wait in the registration area of the Ifo refugee camp which makes up part of the giant Dadaab refugee settlement near the Kenya-Somalia border
"Current humanitarian response is inadequate to meet emergency needs," it said. "Assuming current levels of response, evidence suggests that famine across all regions of the south will occur in the coming one to two months. A massive multisectoral response is critical to prevent additional deaths and total livelihood/social collapse and, most immediately, interventions to improve food access and to address health/nutrition issues are needed."
Andrew Mitchell, the UK's international development secretary, said: "In Somalia, men, women and children are dying of starvation. The fact that a famine has been declared shows just how grave the situation has become.
"It is time for the world to help but sadly the response from many countries has been derisory and dangerously inadequate. Britain is playing its part, with help for more than 2 million people across the Horn of Africa. Now others must do the same."
A famine is measured by rates of hunger, malnutrition and deaths, but the key to it is that it must be widespread.
Technically, a famine is a mortality rate of more than two people per 10,000 per day; acute malnutrition reaching more than 30%; water consumption becoming less than four litres a day; and intake of kilocalories of 1,500 a day compared with the recommended 2,100 a day.
Hundreds of thousands of people have fled Somalia due to the drought and conflict, and refugees are dying of causes related to malnutrition either during the journey or very shortly after arrival at aid camps. On Sunday, the UNHCR began emergency airlift flights in Nairobi to help hundreds of thousands of Somalis who have taken refuge in neighbouring countries.
© Oli Scarff/Getty ImagesA young boy rests by empty USAID vegetable oil tins in the Dagahaley refugee camp
The UN says nearly half of the population in Somalia is facing a humanitarian crisis and in urgent need of aid. The number of people in crisis has increased by more than 1 million in the last six months. More than 166,000 Somalis have fled the country since the start of the year, with more than 100,000 of those leaving since May.
UN agencies have asked for $1.6bn to pay for essential programmes in east Africa, but have only received half that amount.
Oxfam has accused several European governments of "wilful neglect" over the crisis in east Africa.
The NGO said the international response to the drought in Somalia, Ethiopia and Kenya has been surprisingly slow.
"There is no time to waste if we are to avoid massive loss of life. We must not stand by and watch this tragedy unfold before our eyes," said Fran Equiza, Oxfam's regional director. "The world has been slow to recognise the severity of this crisis, but there is no longer any excuse for inaction."
Oxfam said the UK had led the way in pledging new aid, but said France had failed to match words with any additional funding and that neither Italy nor Denmark had provided any new aid.
"The worst-affected areas have endured decades of marginalisation and economic under-development," said Equiza. "If more action had been taken earlier we would not now be at the stage where so many people are facing starvation."
One in 10 children in parts of Somalia is at risk of starving to death, the International Committee of the Red Cross (ICRC) said last week. The independent aid agency, one of very few with access to Somalia's worst-hit areas, said that even in the Bay and Lower Shabelle regions, Somalia's traditional breadbaskets, nearly 11% of children under five had severe acute malnutrition.
An appeal by the Disasters Emergency Committee, an umbrella group of UK charities, has raised £20m since it launched its east Africa appeal.
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