Food aid for starving Somalis living in Islamist-held territory could be flown into the country within “a week to 10 days”, the World Food Programme said.
The international effort to bring humanitarian relief to 3.7 million Somalis who need urgent help to beat drought and famine is being hampered by al-Shabaab's refusal to let most agencies into their territory.
The al-Qaeda-inspired insurgents backtracked on an earlier promise to allow access.
But the United Nations said it was planning to fly food into areas held by the Islamists despite the ban.
“There are 2.2 million people yet to be reached,” said Josette Sheeran, the head of the agency.
“It is the most dangerous environment we are working in in the world. But people are dying. It’s not about politics, it’s about saving lives now.”
WFP was one of the many organisations that al-Shabaab effectively forced out last year after imposing strict conditions of operation including no foreign female staff.
The group also taxed aid convoys.
Regis Chapman, the head of WFP's operations in Somalia, said that food deliveries would soon start into the limited parts of Mogadishu controlled by the internationally-backed government.
He added that “within a week to 10 days” WFP would be sending food into areas controlled by the Islamists.
The Red Cross on Sunday said that it had delivered 400 tonnes of food to 24,000 people in Gedo province, the first time it had taken supplies into al-Shabaab's territory since 2009. More than 2 million Somalis in the worst affected areas, including two famine zones, live in al-Shabaab territory and cannot be reached by international aid.
They are among more than 11.5 million people in Somalia, Kenya and Ethiopia who need urgent help to keep them from starving after at least two years with no rain.
The UN's Food and Agriculture Organisation will meet with the heads of most major charities in Rome to draw “the political attention of leaders of the world” to the crisis, Cristina Amaral, the FAO's head of emergency operations said