"We are still hoping it will take off today," said WFP spokesman David Orr, noting that the flights had a narrow timeframe to take off, offload food and then return to Nairobi.
"If not it will be going ahead on Wednesday," he said, adding that the flights were waiting only for clearance forms to be completed before taking off.
Programme chief Josette Sheeran said the organisation would begin food airlifts to the Somali capital Mogadishu, as well as aid flights to Dolo in Ethiopia on the border with Somalia and to Wajir in northern Kenya.
The plight of children in Somalia is "the worst I have ever seen," she said, after visiting Mogadishu and the Dadaab refugee camp in Kenya over the weekend.
"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 percent chance, of making it," Sheeran said.
Officials said at Monday's meeting the UN has received about 700 million euros since first launching an appeal for the region in November 2010 but needs a billion more by the end of the year to cope with the emergency.
Charities have voiced disappointment at the international response.
The key challenge for aid groups has been reaching parts of southern Somalia held by the Al Qaeda-inspired Islamist group Shebab, which has banned WFP and other international humanitarian agencies from operating on its territory.