Tens of thousands of Somalis are feared dead in the world's worst famine in a generation, the United Nations has said.
The US said it would allow emergency funds to be spent in areas controlled by al Qaida-linked militants as long as the fighters did not interfere with aid distribution.
Exhausted, stick-thin women are stumbling into refugee camps in Kenya and Ethiopia with dead babies and bleeding feet, having left weaker family members behind along the way.
"Somalia is facing its worst food security crisis in the last 20 years," said Mark Bowden, the UN's top official in charge of humanitarian aid in Somalia.
"This desperate situation requires urgent action to save lives. It's likely that conditions will deteriorate further in six months."
The crisis is the worst since 1991-92, when hundreds of thousands of Somalis starved to death, Mr Bowden said. That famine prompted intervention by an international peacekeeping force, but it eventually pulled out after two American Black Hawk helicopters were shot down in 1993.
Since then Western nations have mainly sought to contain the threat of terrorism from Somalia - an anarchic nation where the weak government battles Islamic militants on land and pirates hijack ships for millions of dollars at sea.
Oxfam said one billion dollars (£621m) was needed for famine relief. On Wednesday the US announced an additional £17.4 million in emergency funding on top of the £268m million in assistance already given this year. Aid groups have repeatedly called for the lifting of restrictions implemented in 2009 that are designed to keep food and money from being stolen by the uprising.
Susan Rice, the US ambassador to the United Nations, blamed militant group al-Shabab for exacerbating the crisis.
"The reason the aid hasn't gone in sufficient quantities into south and central Somalia is because al-Shabab has prevented those capable of delivering large quantities of aid from having access - and when they have had access they've taxed them, harassed them, killed them, kidnapped them," she said.