"Nearly 3.7 million people are now in crisis," said UN Secretary-General Ban Ki-moon. "We need donor support to address current needs and prevent a further deterioration of the crisis."

The crisis in Somalia - a failing state mired in internal conflict and suffering the worst drought in half a century - has been escalating steadily for months as aid agencies have pleaded for the international community to intervene.

Thousands of Somalis have fled the country in search of food and water, trekking for days under scorching sun toward refugee camps in neighboring Kenya and Ethiopia.

The United Nations uses a five-stage scale to measure hunger. Stage 5, or famine, means that acute malnutrition rates are 30 percent or more, people do not have adequate calorie or water intake and mortality rates are greater than two adults or four children per day per 10,000 people.

After Wednesday's declaration of famine, US Secretary of State Hillary Clinton announced US$28 million in additional funding for the crisis.