The drought and famine in Somalia have killed more than 29,000 children under the age of five, according to U.S. estimates, the first time such a precise death toll has been released related to the Horn of Africa crisis.

The United Nations has said previously that tens of thousands of people have died in the drought, the worst in Somalia in 60 years.

The UN says 640,000 Somali children are acutely malnourished, a statistic that suggests the death toll of small children will rise.

Nancy Lindborg, an official with the U.S. government aid arm, told a congressional committee in Washington on Wednesday that the U.S. estimates more than 29,000 children under the age of five have died in the last 90 days in southern Somalia. That number is based on nutrition and mortality surveys verified by the U.S. Centers for Disease Control and Prevention.

The UN on Wednesday declared three new regions in Somalia famine zones, bringing the total number to five. Out of a population of roughly 7.5 million, the UN says 3.2 million Somalis are in need of immediate lifesaving assistance.

Getting aid to Somalia has been made more difficult because al-Qaeda-linked militants control much of the country's most desperate areas. Al-Shabab has denied that a famine is taking place, and won't give access to the World Food Program, the world's biggest provider of food aid.

Tens of thousands of refugees have fled south-central Somalia in hopes of finding food at camps in Ethiopia, Kenya and in Mogadishu, the Somali capital.

Hundreds of millions of dollars have been donated to fight the hunger crisis, but the UN says it needs hundreds of millions more.