"It is unacceptable for more than 12 million people to be at risk of starvation today," Jacques Diouf, head of the Food and Agriculture Organisation, said at the start of a conference on the drought crisis in Rome.

"The required funding is lacking. If governments and their donor partners do not invest now, the appalling famine we are now struggling to redress will return to shame the international community yet again," he said.

Diouf called for immediate food aid to help the worst-affected in the region but also for longer-term assistance to livestock farmers and to crop producers to help strengthen their defences against the impact of climate change.

"Although aid is slowly flowing to affected areas and short-term needs are gradually being met, we must start now to help people build a future," he said.

Drought "is not new to the region but is most likely to increase in intensity, severity and frequency in the coming years owing to climate change," he added.

UN representatives and government officials were meeting in Rome after the United Nations said this month that only half of the funds required to deal with a crisis that has hit Djibouti, Ethiopia, Kenya, Somalia, Sudan and Uganda.

At an emergency meeting in Istanbul on Wednesday, the Organisation of Islamic Cooperation vowed to donate 350 million dollars for famine relief in Somalia. More pledges are expected at a donor conference in Ethiopia next week.

The United Nations has officially declared famine in parts of Somalia for the first time this century, including in Mogadishu and four southern regions.

It is the worst crisis in Somalia since 1991-1992, according to the UN.

The Food and Agriculture Organisation warned efforts to tackle the emergency were being hampered by lack of donor funds, with FAO chief Jacques Diouf saying the sight of 12 million people at risk of starvation was "unacceptable".

"The required funding is lacking," Diouf said at a conference in Rome.

"If governments and their donor partners do not invest now, the appalling famine we are now struggling to redress will return to shame the international community yet again," he told the assembled UN officials and ministers.

Diouf called for immediate food aid to help the worst affected in famine-stricken Somalia but also emphasised the need for assistance for livestock farmers and crop producers to resist the effects of climate change in the region.

The World Health Organisation and the UN children's agency UNICEF meanwhile warned that outbreaks of cholera and acute diarrhoea were on the rise in Somalia, with 4,272 cases reported at one hospital in the capital Mogadishu alone.

Marthe Everard, WHO's Somalia representative, said: "We urgently need more mobile clinics that will provide basic health care services to the many displaced and who will strengthen the reporting on new outbreaks. This is critical."

The United Nations says it has received pledges for $1.3 billion of the $2.4 billion (1.7 billion euros) required for relief efforts in the Horn of Africa.

The African Union is holding a donor conference in Ethiopia next week.

"The time for talk is over," said Penny Lawrence, international director for Oxfam, which on Thursday began airlifting 47 tonnes of aid supplies into Mogadishu.

"The clock is ticking for East Africa, and the African Union governments who are meeting next week must show their determination to make this Africa's last famine," she said.

The United Nations has officially declared famine in parts of Somalia for the first time this century, including in Mogadishu and four southern regions controlled by Islamist militias who have blocked access for some aid groups.

Following a trip to Somalia this month, Cristina Amaral, FAO's head of emergency operations, said the most urgent tasks were to prepare farmers for the October rainy season and widen humanitarian access in southern Somalia.

"There is a lot still to be done but above all we want to alert people to the fact that we have to prepare for the next farming season," she told AFP.

Tens of thousands have died and hundreds of thousands have fled in a crisis that has also hit Djibouti, Ethiopia, Kenya, South Sudan, Sudan and Uganda.

"Emergency assistance alone will not solve the underlying problems in the country," Somali Agriculture Minister Abdullahi Haji Hassan told the conference.

"What is also needed is a long-term sustainable approach," he said.

Kenyan Agriculture Minister Sally Kosgei complained that programmes to help farmers "do not seem to be taken seriously" by the international community.