Agency director Josette Sheeran told the Voice of America the first flights will land in the capital, Mogadishu, and others will deliver aid to points in eastern Ethiopia and northern Kenya near the Somali border.
The United Nations also has called a donors conference for Wednesday in Nairobi, Kenya, aiming to raise $1.6 billion to address the famine over the next year.
On Tuesday, U.N. Secretary General Ban Ki-moon asked leaders of four Persian Gulf states to pledge more assistance, and King Abdallah of Saudi Arabia promised an extra $50 million, Sheeran said.
The U.N. officially declared a famine in southern Somalia last week, and more than 11 million people are in desperate need of emergency aid. Sheeran said a third of the Somali population is facing starvation.
The U.N. High Commissioner for Refugees said Tuesday people are streaming into camps around Mogadishu at the rate of 1,000 daily.
Also Wednesday, UNICEF said it must vaccinate more than 300,000 children in Kenya to stem epidemics spread by the refugee flow.