He called on countries to back a call by the 53-nation African Union (AU) for support to expand its peacekeeping force in the region from 3,200 to more than 7,700 troops.

Annan, however, stressed that only a political solution would bring an end to what the UN has described as the world’s worst humanitarian crisis.

"We are running a race against time," he said in a speech at a donor-pledging conference at the AU headquarters in Addis Ababa. "The rainy season and the 'hunger gap' are approaching fast, making our relief operations more difficult just as they need to expand further.

"If violence and fear prevent the people of Darfur from planting and growing crops next year, then millions will have to be sustained by an epic relief effort which will stretch international capacity to the maximum," he added.

The AU told donors it needed US $466 million to more than double its force in Darfur by September and equip it with attack helicopters, armoured personnel carriers and fuel.

An expanded mission of 12,300, which could be on the ground early next year, would cost in the region of $723 million, the AU added. Nigeria, Rwanda, Kenya, South Africa, Gambia and Senegal have all said they have troops ready.

Annan urged rich nations to fund an expansion of the force, saying civilians and aid workers were being attacked.

"Although the violence in Darfur has stabilised over the last few months, the situation remains unacceptable," he said. "Civilians are still at risk and subject to attacks. Moreover, the violence is increasingly targeted at aid workers, hampering their difficult work."

Annan also appealed for money to fund UN humanitarian activities in Sudan, where food distribution by the World Food Programme (WFP) faces a chronic shortage of funds.

"More than three million [people] will need relief to get through the next few months," he said. "We are still $350 million short of what we require to provide that relief."

He urged both rebel groups and the government to resume AU-mediated talks, which stalled in December but are due to resume in Abuja, Nigeria, in June.

The Sudanese vice-president, Ali Osman Taha, said that continued ceasefire violations by rebel groups could threaten the peace talks.

"The future of the talks depends greatly on the building of peace and respect of the ceasefire," Taha said. "We want a clear message to be sent so that the rebel factions respect the ceasefire agreement and stop attacks against the humanitarian convoys and civilian populations."

According to aid workers, at least 180,000 people have died and about two million others have fled their homes in Darfur to escape the conflict between rebels on one side and pro-government militia on the other.

The AU's role in bringing peace to the region is seen as a "critical test" of its new peacekeeping abilities.

The international community acknowledged it was under the spotlight to provide support to the AU. The US pledged an additional $50 million, while Canada offered $150 million. Javier Solana, the EU’s foreign-policy chief, pledged training for police and troops and airlifting of troops into hot spots.

Jaap de Hoop Scheffer, NATO Secretary-General, told the conference that the AU must remain in control.

"The AU has to remain in the driving seat to solve this difficult conflict," he said. "Much is at stake. Neither the AU or the international community can afford failure in Darfur."

NATO offered air support, training and intelligence.