The UN World Food Programme is to start airlifting food to Somalia, WFP head Josette Sheeran said at crisis talks on East Africa's drought.
This will be the first airlift of food aid since the UN declared a famine in two areas of Somalia last week.
Somali Foreign Minister Mohamed Ibrahim warned at the emergency meeting in Rome that more than 3.5 million people "may starve to death" in his country.
Islamists, who control most of Somalia, have banned the WFP from their areas.
Al-Shabab, which has ties to al-Qaeda, has accused the groups it has banned from its territories of being political.
Ms Sheeran said aid would be airlifted to the capital, Mogadishu, where the weak interim government - backed by an African Union peace force - controls only parts of the city.
Tens of thousands of Somalis have been fleeing al-Shabab areas and heading to Mogadishu and neighbouring Kenya and Ethiopia in search of food.
Ms Sheeran attended the meeting in Rome - called by France, which chairs the G20 group of developed and emerging economies - after visiting Mogadishu and the Dadaab refugee camp in Kenya.
"What we saw is children who are arriving so weak that many of them are in stage four malnutrition and have little chance - less than 40% chance - of making it," she said.
"This is about saving lives now. It's not about politics, it's not about anything but humanity standing together to save lives."
Mr Ibrahim said that food aid was mostly needed in areas controlled by al-Shabab.
"The UN estimates that more than 3.5 million Somalis, the vast majority of them in the insurgent-held areas, may starve to death," the foreign minister said.
But the director of the international charity Goal, John O'Shea, told the BBC the UN's response to Somalia's political crisis had worsened the crisis.
He said the UN Security Council should have authorised a sizeable force of peacekeepers to end years of conflict in Somalia.
"We wouldn't have four million Somalis starving if they sent in UN peacekeepers," he told the BBC's Focus on Africa programme.
Somalia has 9,200 African Union peacekeepers out of a promised 20,000 - all of them based in Mogadishu.
'Scandal of century'
France's Agriculture Minister Bruno Le Maire told the meeting at the UN's Food and Agriculture Organisation headquarters in Rome that the world had "failed to ensure food security".
"If we don't take the necessary measures, famine will be the scandal of this century," AFP news agency quotes Mr Le Maire as saying.
Ahead of the Rome summit, the World Bank pledged $500m (£307m) to help.
Some $12m will be for immediate assistance to those worst hit by what the UN says is East Africa's worst drought in 60 years.
But the bulk of the money will go towards long-term projects to aid livestock farmers.
UN chief Ban Ki-moon has urged donor nations to supply an extra $1.6bn and a donors' conference is due to take place in the Kenyan capital, Nairobi, on Wednesday.
Somalia is thought to be worst-hit by the crisis, but Ethiopia, Kenya and Djibouti have also been affected.
More than 10 million people in the region are thought to be at risk of starvation.
Analysts say the drought has been caused by the lack of rains and the failure of governments to adequately finance agriculture and irrigation schemes.
