Famine is declared when acute malnutrition rates among children exceed 30 per cent and more than two people per every 10,000 die per day.
An al-Qaeda-linked militant group has said it will not allow banned aid organizations to return, meaning only a handful of agencies will be able to respond to the worsening famine in southern Somalia.
Meanwhile, the UN children's agency - one of the few groups that does operate in the area - said Friday 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 neighboring 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 late Thursday that aid agencies the group previously banned are still barred. A statement from Mr. Rage earlier this month had said that the group wanted to open talks with aid groups to facilitate their return.
Some groups, like UNICEF and Save The Children, operate in militant- controlled areas of Somalia. But other groups, like the UN's World Food Program and Mercy Corps, are banned.
Mr. Rage also called the UN's declaration of famine in parts of Somalia politically motivated and "pure propaganda."
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 Program said Friday it will begin providing food for 175,000 people in the Gedo region of southwest Somalia and to 40,000 people in the Afgoye corridor northwest 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.
The global body estimates that more 11 million people in East Africa are affected by the drought, with 3.7 million in Somalia among the worst-hit because of the ongoing civil war in the country.
WHO's representative for Somalia warned Friday that the conditions for declaring a famine are expected to be reached soon in two further parts of southern Somalia - Juba and Bay.
Melissa Fleming, a spokeswoman for the UN refugee agency, said waiting until people cross into neighboring Ethiopia and Kenya would mean many Somali women and children starving to death before they reach the safety of camps there. Already malnutrition rates among new arrivals in Ethiopia are among the worst ever seen, UN officials said.