Jill Biden, Vice President Joe Biden's wife, visited a refugee camp to raise awareness about the issue.
She asked Americans and people around the world to reach out to help.
A drought has turned into famine because little aid can reach militant-controlled south-central Somalia.
Washington is preparing to announce roughly $100 million in new famine aid, two U.S. officials said.
More than 29,000 children under the age of 5 have died in the last 90 days in southern Somalia alone, according to U.S. estimates. The U.N. says 640,000 Somali children are acutely malnourished, suggesting the death toll of small children will rise.
Aid is only reaching about 20 percent of the 2.6 million Somalis who need it, Mark Bowden, the U.N.'s top humanitarian official for Somalia, said on a visit to Mogadishu on Monday. He cited transport and security as the two main problems.
The Associated Press contributed to this report.