Hundreds of thousands of Somali children could die in the country's famine unless more help arrives, a top U.S. official said Monday as Washington prepares to announce $100 million in new famine aid.

To highlight the crisis, the wife of Vice-President Joe Biden visited a refugee camp on a patch of desert in eastern Kenya where tens of thousands of Somalis have massed. A drought has turned into famine because little aid can reach militant-controlled south-central Somalia.

Jill Biden, wife of U.S. Vice-President Joe Biden, listens to a Somali refugee at a UNHCR screening centre on the outskirts of Ifo camp outside Dadaab, eastern Kenya. Jerome Delay/Associated Press

Jill Biden is the highest-profile U.S. visitor to East Africa since the number of refugees coming across the Somali border dramatically increased in July. Biden arrived in a C-130 military transport plane and said she wants to raise awareness and persuade donors to give more.

"What I'm asking is for Americans to reach out and help because the situation is dire," said Biden, who met with two Somali mothers and their eight children. "There is hope if people start to pay attention to this."

More than 29,000 children under the age of five have died in the last 90 days in southern Somalia alone, according to U.S. estimates. The UN says 640,000 Somali children are acutely malnourished, suggesting the death toll of small children will rise.

USAID administrator Raj Shah, who accompanied Biden, said hundreds of thousands of children could die from the famine.

U.S. officials said Washington is set to announce an aid package of about $100 million for famine relief efforts. The officials could not be quoted by name ahead of a formal announcement.

More than 12 million people in the Horn of Africa are in need of immediate food aid, including nearly half of Somalia's population. The UN has declared five famine zones in Somalia, including the camps for displaced people in Mogadishu, the capital.

Aid is only reaching about 20 per cent of the 2.6 million Somalis who need it, Mark Bowden, the UN's top humanitarian official for Somalia, said on a visit to Mogadishu on Monday. The situation is better in the Somali capital, where about half the city's 600,000 inhabitants are receiving aid, he said.

Transport and security are the two main problems, he said, and it is unclear what the effect will be of the withdrawal of Islamist insurgents from their bases in the capital on Saturday. There have been several serious gunfights at aid distributions recently, and at least 10 people have been killed.

"An absence of conflict does not mean that there is security here," he said. "There's always been factions and militias."

A senior U.S. official travelling with Biden said the U.S. believes it is too early to tell what al-Shabab's intentions are, but that the reported withdrawal could be a hopeful sign that more aid could soon reach those in need.

Kiki Ghebo, another top UN humanitarian official, said different kinds of aid is needed: food for the starving, vaccines and medical help to prevent disease outbreaks, and things like plastic sheeting and cooking utensils for those who had been forced to flee their homes because of the war and famine.