The United Nations says four million people are now in crisis as famine spreads in Somalia – and 750,000 are at risk of death.

The U.N. reported Monday that famine has extended to a sixth region in southern Somalia and is expected to spread further in the next four months unless aid is increased.

The world body's Somalia Food Security Nutrition Analysis Unit says 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 international organization says tens of thousands of people have already died, half of them children.

In the report, the U.N. calls for a “massive” multi-pronged response to prevent additional deaths and social collapse. Most immediately, improved food access and help with health and nutrition are needed. The UN says aid will continue to be necessary through 2012.

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.