The World Food Program called today for an investigation into allegations that the Sudanese government is using chemical weapons against rebel forces.

The U.N. relief agency's suspicions were aroused this week when two of its workers felt a burning sensation and began retching during a trip through the rebel-controlled province of western Equatoria.

"We are taking this very seriously," said Lindsey Davies, a spokeswoman for the World Food Program in Nairobi, Kenya.

U.N. officials said two relief workers, from Britain and Uganda, and a Sudanese escort were returning from a food delivery on July 27 when they stopped for 45 minutes at a police checkpoint in the town of Lainya, four days after it was attacked from the air.

"There was a very bad smell in the area coming from a bomb crater 10 meters away," and the men felt "a burning sensation in their noses and eyes," Davies said. Then they began coughing, and all three soon developed headaches, nausea and vomiting, she said.

The civil war in Sudan has dragged on for more than 16 years, leaving 1.5 million dead from violence and famine. The Sudan People's Liberation Army (SPLA), made up primarily of Christians and animists from southern Sudan, has been fighting the Islamic government in Khartoum since 1983. The latest cease-fire agreement collapsed June 15.

Washington has accused Sudan in the past of developing chemical weapons. Last August, the United States launched cruise missiles at a pharmaceutical factory it suspected of making nerve agents. But so far it has failed to provide compelling evidence that the factory produced chemical weapons.

While experts said some of the symptoms described by the U.N. workers are similar to those caused by mustard gas, they noted that few chemical weapons would maintain their potency for long in Sudan's scorching weather. It is more likely, they said, that smoke bombs or explosives containing white phosphorus are the culprits.

"Nerve agents are referred to as nerve gases, but in fact they are liquids and they evaporate," said Greg Jones, a Rand Corp. expert on chemical weapons. "They would have dissipated in a few hours under the hot Sudanese sun."

The call for a U.N. inquiry comes just days after the SPLA leveled charges that Khartoum has used biological and chemical weapons. On Friday, a spokesman for the SPLA, Samson Kwanje, told reporters that a government Antonov bomber dropped 22 chemical warheads on Lainya and Kaaya.

"We categorically deny these allegations," said Tarik Bakhi, a diplomat at Sudan's mission to the United Nations, adding that his country recently signed the Chemical Weapons Convention. "We don't have anything to hide. I don't think Sudan will mind an investigation."