The United Nations has declared a famine in two parts of southern Somalia, as the Horn of Africa suffers its worst drought in decades.

Speaking in Nairobi Wednesday, U.N. official Mark Bowden said famine exists in Somalia's Bakool and Lower Shabelle regions and warned it could spread if action is not taken to address the crisis.

Bowden, the U.N.'s top humanitarian official in Somalia, also said it is likely that tens of thousands of Somalis have already starved to death.

Bowden said $300 million in aid is needed during the next two months in Somalia alone.

U.S. Secretary of State Hillary Clinton announced Wednesday that Washington will add $28 million in aid to the more than $431 million in assistance it has already provided for the region.

The United Nations has said more than 11 million people across East Africa need emergency assistance because of severe drought.

The U.N. Food and Agricultural Organization is appealing for $120 million in aid for the Horn of Africa from the international community.

More than half of the aid would go towards helping Somalia. The remaining funds would be designated for use in Ethiopia, Kenya and Uganda.

The U.N. declares a famine when malnutrition rates are higher than 30 percent.