The United Nations says famine has spread to a sixth region in southern Somalia.

The world body's Somalia Food Security Nutrition Analysis Unit said Monday that acute malnutrition and the rate of crude mortality have surpassed famine thresholds in the country's Bay region.

The report says poor households in the region face massive food deficits as a result of poor crop production and deteriorating purchasing power.

The U.N. says 4 million people are in crisis in Somalia, with 750,000 Somalis at risk of death in the coming four months in the absence of an adequate aid response to the famine and drought. The international organization says tens of thousands of people have already died, half of them children.

The U.N. says the famine is expected to spread further in Somalia over the next four months.

The extreme drought has also affected parts of Kenya, Ethiopia, Eritrea and Djibouti.

The U.N. has said $2.5 billion is needed for drought and famine relief.

The African Union said last month at a donors conference that it has raised more than $350 million in cash for drought and famine relief. The AU has been criticized for its slow response to the crisis, which has forced hundreds of thousands of Somalis to flee their homes in search of food and water. Before the conference, the 54-member bloc had pledged only $500,000 in aid.

The United States has pledged more than $500 million in food and refugee assistance. The European Union and its member countries have pledged more than $750 million, while the Organization of the Islamic Conference has pledged $350 million.