Somalis are fleeing to Ethiopia, Kenya and the capital, Mogadishu, in search of food The UN has made its first aid delivery to drought victims in areas of Somalia controlled by al-Qaeda-linked militants since they lifted an aid ban.UN children organisation's Rozanne Chorlton said al-Shabab had given UN workers unhindered access and hoped this would encourage other agencies.
It comes as the UK pledged ?52.25m ($84m) in emergency drought aid.
But the UK's overseas aid minister told the BBC the UK would not deal with al-Shabab, which controls much of Somalia.
Andrew Mitchell is touring the huge Dadaab camp in north-eastern Kenya to see the scale of the crisis caused by the drought, the Horn of Africa's worst in 60 years which is estimated to be affecting some 10 million people.
'Close to famine'Unicef airlifted food and medicine to malnourished children to the central town of Baidoa, more than 200km (about 125 miles) north-west of the capital, Mogadishu.
Ms Chorlton, the Unicef representative for Somalia, said al-Shabab had assured the agency it could operate without undue interference.
Andrew Mitchell: "Britain is putting its shoulder to the wheel... to stop this becoming a catastrophe"Al-Shabab, which rules over large swathes of south and central Somalia, had imposed a ban on foreign aid agencies in its territories two years ago, accusing them of being anti-Muslim. It lifted the ban 10 days ago as long as groups had "no hidden agenda".
"They gave assurances that our access for humanitarian purposes would be unhindered and that we would be able to reach the people who need support most," Ms Chorlton told the BBC.
Unicef paid no fees to al-Shabab, and that the success of the mission meant it would be repeated in the near future, she added.
She warned the situation was close to famine.
Thousands of people have been fleeing al-Shabab's territories in search of food and water - some to Mogadishu, where aid agencies are operating in areas controlled by the the weak interim government, and others to Ethiopia and Kenya.
Some 1,400 Somali refugees are arriving every day at Kenya's overcrowded Dadaab camp - some walking up to 20 days to get there.
'Catastrophe'Mr Mitchell, the UK's international development secretary, estimates that there are about 400,000 people in the camp. Aid agencies fear numbers could rise to half a million.
"More than 3,000 people every day are fleeing over the borders to Ethiopia and Kenya, many of them arriving with starving children," Mr Mitchell said.
"We need everyone who can help from across the world now to make sure they focus on this developing crisis here to stop it becoming a catastrophe. There is an emergency developing of profound proportions," said the minister.
Continue reading the main story
Extended drought is causing a severe food crisis in the Horn of Africa, which includes Kenya, Ethiopia, Djibouti and Somalia. Weather conditions over the Pacific means the rains have failed for two seasons and are unlikely to return until September.
Food shortages are affecting up to 12 million people. The UN has not declared a famine but large areas of the region are now classified as in crisis or emergency, with malnutrition affecting up to 35-40% of children under five. 
The humanitarian problem is made worse by ongoing conflicts, which means that until July militant groups had only allowed aid organisations limited access to large parts of southern Somalia and eastern Ethiopia.
Since the beginning of 2011, around 15,000 Somalis each month have fled into refugee camps in Kenya and Ethiopia looking for food and water. The refugee camp at Dadaab, in Kenya, has been overwhelmed by 370,000 people. 
Farmers unable to meet their basic food costs are abandoning their herds. High cereal and fuel prices had already forced them to sell many animals before the drought and their smaller herds are now unprofitable or dying.
The refugee problem may have been preventable. However, violent conflict in the region has deterred international investment in long-term development programmes, which may have reduced the effects of the drought.
Development aid would focus on reducing deforestation, topsoil erosion and overgrazing and improving water conservation. New roads and infrastructure for markets would help farmers increase their profits. 
The result of climate conditions, conflict and lack of investment is that 6.7 million people in Kenya and Ethiopia are currently existing on food rations, and relief agencies estimate 2.6 million in Somalia will need assistance a new emergency operation. BACK {current} of {total} NEXT "Britain, as always, has shown huge generosity and is in a leadership position to try and resolve this crisis. We need others to do so too. We need the whole of the international community now to bend every sinew to help these poor people here who are in a desperate condition."He said the UK's aid package would also help victims of the drought who remain in Somalia, which has been racked by constant war for more than 20 years - its last functioning national government was toppled in 1991.
"We simply will not deal with al-Shabab and we will not allow our operations to be fettered by them," said Mr Mitchell.
"We must be able to see that it can actually reach, with lifesaving provision, those for whom it is intended, then we will be giving additional support inside Somalia now and scaling that up."
The UK's ?52.25m aid package comes after a joint charity appeal by the Disasters Emergency Committee (DEC) saw more than ?13m raised in a week.
The cash is in addition to the ?38m food aid package announced on 3 July to feed 1.3m people for three months.
The Department for International Development (Dfid) said the money would help:
500,000 people in Somalia, including treatment for nearly 70,000 acutely malnourished childrenMore than 130,000 people in the Dadaab camps to help provide them with clean drinking water and health care100,000 people in Dolo Ado refugee camps in Ethiopia to provide them with shelter and clean drinking water as well as targeted treatment of starving children300,000 Kenyans, including special rations to prevent malnutrition in children under the age of five and breastfeeding mothers.
Somali refugees have trekked into Kenya after the worst drought in the region for 60 years British aid agencies are preparing to expand their activities in Somalia to help some of the 10 million people at risk of starvation in East Africa.
Ugandan troops make up the majority of AU peacekeepers fighting Islamist militants in Somalia Ugandan President Yoweri Museveni has said Somalia's transitional government should be given another year to consolidate gains against militants.