The harsh drought ravaging the Horn of Africa is likely to worsen and could trigger famine in more regions of Somalia, aid groups warned Tuesday as they struggle to help the millions already affected.

The United Nations last month declared famine in the Bakool and Lower Shabelle regions in the south of Somalia, but rising food costs, poor or no harvests and devastated livelihoods were worsening the effects of the drought.

"The next three to four months are set to worsen in Ethiopia, Kenya and parts of southern Somalia and the situation will remain classified as an 'emergency' until the end of the year," aid agency Oxfam warned Tuesday.

"The whole south of Somalia is likely to be declared a famine due to a combination of worsening pastoral conditions, further food price increases and poor harvest."

An extra $1.4 billion is needed to stop the spread of famine, said Valerie Amos, the UN under secretary general and emergency relief coordinator. Some $2.48 billion is required to help the drought victims.

Around 12 million people in the Horn of Africa are in danger of starvation because of the prolonged drought that has also hit parts of Ethiopia, Kenya, Djibouti and Uganda.

The UN Food and Agricultural Organisation said Uganda could soon fall victim of severe food shortages.

"We have started to monitor the situation in Uganda where we are also seeing pockets of food insecurity affected by the same drought conditions," a FAO spokeswoman said.

"Uganda may be the next country hit with these same sort of alarming malnutrition and drought conditions."

Relief groups are struggling to deliver aid to the drought victims and have urged donors to step up assistance to ease the suffering brought by Horn of Africa's worst drought in decades.

The UN children's agency UNICEF appealed to airlines to help fly in food urgently to the drought-hit area.

"We're making an appeal to the air transport industry for free and heavily discounted cargo space to help us transport emergency nutrition supplies to the Horn of Africa," said Marixie Mercado, a UNICEF spokeswoman.

British Airways, Lufthansa, Virgin Atlantic and UPS have all offered to transport between 15 and 50 metric tonnes a week, but UNICEF is "looking to others to step forward as well," she said.

UNICEF is trying to get 5,000 metric tonnes of food from its warehouses in Europe to the region, but it costs about $350,000 (250,000 euros) to transport just 100 metric tonnes of food by cargo jumbo jet from France to Nairobi -- and shipping the food would take six weeks.

On Tuesday, US officials said Washington was easing sanctions related to Somalia's Al-Qaeda-linked Shebab rebels to boost assistance to the drought victims.

The officials said sanctions against the militia would be maintained, but the United States will not prosecute any aid group that makes "good-faith efforts to deliver food."

"Our number one goal is to save lives. Time is not on our side," a US official said on condition of anonymity.

Another official said: "In the face of this evolving crisis and the extreme humanitarian need, we have issued new guidance to allow more flexibility."

The Shebab rebels control much of southern and central Somalia. Two years ago, they banned several foreign aid groups from regions under their control but allowed a handful of others to operate.

However their work is still limited by the insurgents' restrictions.

Somali President Sharif Sheikh Ahmed criticised Tuesday a lack of coordination by African and Arab donors to aid millions of drought-stricken people in his country, the worst affected in the region.

But despite the criticism, his government's authority is limited to just a few areas in the capital Mogadishu where it survives under the protection of African Union forces.