Sudanese authorities are holding up to 100,000 tonnes of sorghum meant for Darfur, alleging that it is genetically modified, the UN food agency said on Wednesday.
The sorghum, which comes from the United States, is being held up at Port Sudan, a World Food Program spokeswoman in Rome said, adding that laboratory tests had shown it was not genetically modified.
"We had it tested by a French laboratory along with Canadian split peas which the Sudanese are also objecting to, and neither food consignment is GM.
In any case, there is no GM sorghum on the market, it doesn't exist," said the WFP's Caroline Hurford.
"It's a huge amount of food to be held up and our sub-offices in Darfur must be getting quite worried. We hope that we can find a solution quickly and have it released so it can reach the people who need it," she said.
Sources in the aid community said Sudan was blocking the shipment because it wanted the WFP to buy local food products.
"We do intend to buy some amount of food from Sudan as they had a bumper harvest but there are limits to how much we can purchase because of funding. Most food aid is given to us in kind, as is the case with the US sorghum," Hurford said.
Harry Edwards, a spokesman for the US Agency for International Development in Washington, which manages most US food aid donations, said only that the crop provided to the WFP contained no genetically altered material.
The sources said Khartoum also may be using the food aid as a political weapon as international pressure intensifies on the government to stop the violence in Darfur.
The United Nations estimates around 200,000 people have been killed in the vast western region and 2.5 million displaced since 2003, when rebel groups took up arms against the government, accusing it of neglect.
Human rights groups say the Sudanese government armed Arab militias to help quell the rebellion. Khartoum denies the charge.
The United States, which has called the violence "genocide," has accused the Sudanese government of a campaign of intimidation against aid workers in Darfur, and threatened Khartoum with sanctions and other punitive measures.
A decision is expected within weeks.
The WFP's new head, Josette Sheeran, began a visit to Sudan on Wednesday, her first field trip as executive director of the agency. Sheeran is a former under-secretary for economic affairs at the US State Department.