Some 10 million people are in dire need of humanitarian assistance as East Africa struggles with the worst drought in decades.
Large areas of Kenya, Somalia, Ethiopia are affected by the drought conditions.
Many people have left their homes with children to trek barefoot for days across parched scrubland in the hope of finding food and water.
In Somalia, Islamist rebels have lifted a ban on humanitarian agencies supplying food aid to millions of people affected. Somalia is experiencing pre-famine conditions, driving more than 1,000 people over the border into Kenya and Ethiopia each day, according to the United Nations.
The UN says 2.8 million people in Somalia need emergency aid. In the worst-hit areas, one in three children is suffering from malnutrition.
"We have now decided to welcome all Muslim and non-Muslim aid agencies to assist the drought-stricken Somalis in our areas," said al-Shabaab spokesman Sheikh Ali Mohamud Rage in Mogadishu earlier this week.
"All aid agencies whose objective is only humanitarian relief are free to operate in our area," he said, adding they should first contact al Shabaab's drought committee.
Al-Shabaab fighters, who profess loyalty to al-Qaeda, control central and southern parts of the country. In the past, they said food aid created dependency.
In Washington, Secretary of State Hillary Clinton told officials yesterday to step up cooperation with regional governments to head off a humanitarian disaster and test al-Shabaab's willingness to allow food aid in, the State Department said.
"Obviously it's incumbent on us now, all of the aid donors and the international community, to test whether they really are ready to let starving people get humanitarian aid," one senior US official said. "The instruction this morning was: this is very serious. We've started to help, let's see what else we can do."
The UN World Food Programme pulled out of southern Somalia in 2010 because of threats against its staff and demands by al-Shabaab of payments for security.
According to Britain’s Disasters Emergency Committee (DEC) which co-ordinates responses to major disasters overseas, more than 1,300 people - the majority of them children - are arriving in the Dadaab refugee camp in eastern Kenya near the border with Somalia every day.
The camp is already believed to be the largest of its kind in the world with a population of around 350,000.
Agencies