An al Qaida-linked militant group has said it will not allow banned aid organisations to return to southern Somalia, meaning only a handful of agencies will be able to respond to the worsening famine.
Meanwhile, the UN children's agency - one of the few groups that does operate in the area - said today that nearly 800,000 children are at risk of dying without urgent assistance.
The UN says it fears tens of thousands of people already have died in Somalia's famine, which has prompted Somalis to walk for days in hopes of reaching a refugee camp in neighbouring Kenya.
"Unicef is using every means possible to reach every child," said Elhadj As Sy, a regional director for Unicef. "Every life must count and we cannot afford to lose more lives to this crisis."
The spokesman for the militant group al-Shabab, Sheik Ali Mohamud Rage, said last that aid agencies the group previously banned are still barred.
A statement from Rage earlier this month had said that the group wanted to open talks with aid groups to facilitate their return. Rage also called the UN's declaration of famine in parts of Somalia politically motivated and "pure propaganda."
Some groups, like Unicef and Save The Children, operate in militant- controlled areas of Somalia. But other groups, like the UN's World Food Programme and Mercy Corps, are banned.
Somalia's prolonged drought devolved into famine in part because neither the Somali government nor many aid agencies can fully operate in areas of southern Somalia controlled by al-Shabab.
The World Food Programme said it will begin providing food for 175,000 people in the Gedo region of south-west Somalia and to 40,000 people in the Afgoye corridor north-west of the capital Mogadishu. The UN food agency also plans airlifts of aid to Mogadishu, WFP spokeswoman Emilia Casella told reporters in Geneva.
On Wednesday, the UN declared a famine in the Bakool and Lower Shabele regions of southern Somalia, greatly raising the profile of what has been a steadily worsening food crisis in the Horn of Africa.