The UN food agency on Wednesday staged a rare air drop of wheat to thousands of desperate refugees in South Sudan stranded by heavy rains, officials said.

Some 32 tons of wheat were flown from a base in Ethiopia to refugee settlements in South Sudan where hundreds of thousands of people have fled fighting in Sudan's South Kordofan and Blue Nile states, the World Food Programme (WFP) said.

"This is the first in a series of air drops that aims to replenish rapidly diminishing food stocks for more than 100,000 families who have fled fighting north of the border," said WFP executive director Ertharin Cousin in a statement.

The first supply will be enough to feed 2,100 people for one month, according to Cousin, who is in South Sudan.

Many refugee camps are cut off because it is the rainy season in South Sudan. The deliveries had to be made by air because South Sudan, which broke from Sudan in July last year and is one of the world's poorest nations, has barely 100 miles (160 kilometers) of paved road.

Rebels have been fighting Sudanese government troops in Kordofan and Blue Nile for more than a year, sparking the refugee exodus across the border. Sudan's government has severely restricted international access to the conflict zone.