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.
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.
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.
"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 children
- More than 130,000 people in the Dadaab camps to help provide them with clean drinking water and health care
- 100,000 people in Dolo Ado refugee camps in Ethiopia to provide them with shelter and clean drinking water as well as targeted treatment of starving children
- 300,000 Kenyans, including special rations to prevent malnutrition in children under the age of five and breastfeeding mothers.