"Already rains have begun in the southern most parts of Darfur," the USAID-funded Famine Early Warning System Network (FEWS Net) said in a report released on 5 May. It detailed how rainfall would affect relief efforts in the crisis-hit region of western Sudan.

"Northern areas, like El Fasher will start to experience heavy seasonal rains by the end of June," the report added. "By the end of July the rains will cover the entire region."

FEWS Net warned that once the seasonal rains started, much of eastern Chad, where thousands of Sudanese refugees live, would also be cut off. While large towns in Darfur could remain accessible, surrounding areas would become difficult to reach.

The pre-season rainfall could result in 20 mm to 50 mm of early precipitation in parts of West and South Darfur. The report estimated that the rains would affect the southernmost camps for internally displaced persons in the region.

"While the rainy season is upon us, it builds up slowly and it takes a while before the roads become impassable," Rene McGuffin, spokeswoman for the United Nations World Food Programme (WFP), told IRIN on Monday.

On Saturday, WFP began airlifting food from a Libyan airport directly into Darfur, pioneering a new route to transport food to nearly two million people during the rainy season.

"The extra capacity using the Al Kufra airlift will be a tremendous help during the approaching rainy season and concurrent period of greatest food shortages," Ramiro Lopes da Silva, WFP representative and country director in Sudan, said in a statement on Saturday.

"We are looking at a worst-case scenario of more than three million people needing food assistance in Darfur from August," he added.

The airlift was the first time food aid had been transported via Libya directly to Darfur. It followed the opening up last year of an ancient overland caravan route for food convoys from Libya to refugee camps in Chad.

The new air corridor could make it possible to deliver an extra 5,000 mt of food each month to Darfur.

"Most secondary and tertiary roads become cut off in July and August, and even the main roads can become very difficult to pass," McGuffin said. "Last year, West Darfur in particular was a real struggle in terms of accessibility," she noted.

Meanwhile, the UN Mission in Sudan reported that its staff met with the African Union (AU) in Nyala on Sunday to discuss the migration routes of nomadic tribes who were expected to herd four million heads of livestock from the south to the north during the start of the rainy season.

This migration corridor, identified by the Sudanese government, would be monitored by the AU. The pan-African body would also liaise with armed groups in the region to minimise violent clashes during the migration process.

The war in Darfur pits Sudanese government troops and militias - allegedly allied to the government - against rebels fighting to end what they have called marginalisation and discrimination of the region's inhabitants by the state.

More than 2.4 million people continue to be affected by the conflict in Sudan, 1.85 million of whom are either internally displaced or have been forced to flee to neighbouring Chad.