UNITED NATIONS — Refugee camps in the Sudanese region of Darfur are being infiltrated by Al Qaeda, Chad said yesterday.

Backed by wealthy Saudi Arabians and the Sudanese government, mercenaries are gaining power in Darfur, Chad said. A neighbor of Sudan, Chad is strongly advocating a U.N. force for Darfur, though Sudan rejected a U.N. Security Council proposal yesterday to boost the small African Union force now in the region.

The U.N. proposal would have added more troops and equipment for a new force operating under the command of the United Nations. Chad hosts millions of Sudanese refugees who have crossed the border from Darfur to escape the murder, rape, and pillaging committed by Sudanese government-backed Arab militias known as the janjaweed, in a campaign described by the Bush administration as genocide.

When American officials offered recently to involve NATO in stopping the violence in Darfur, Sudanese officials said the presence of Westerners in their mostly Muslim country would only inspire the militants, including Al Qaeda, who would unite to fight the foreigners.

But the Chadian ambassador to the United Nations, Mahamat Ali Adoum, wrote to the president of the Security Council yesterday that unless foreign troops are sent to control the refugee camps, Al Qaeda will join forces with anti-government rebels there.

According to Mr. Adoum’s letter, Chad is “again the victim of a major operation of destabilization from neighboring Sudan.”

Khartoum, he added, is “sending mercenaries and others to attack the positions of our security forces” in eastern Chad, near the border with Darfur.

“Strangely enough,” he added, “this new operation also bears the mark of Saudi Arabia, or at least of certain influential circles close to the royal family, which are helping to recruit and equip the young mercenaries linked to the mysterious al Qaeda,” Mr. Adoum wrote.

The State Department yesterday expressed concern that in addition to the western region of Darfur, fighting has begun again in southern Sudan, where a U.N. force is charged with holding together a fragile power-sharing agreement.

After months of Security Council negotiations, President al-Bashir of Sudan yesterday reverted to his previous rejection of any Darfur force under U.N.command. “We can take technical, advisory, and financial support from the U.N., but no U.N. force,” he told an African Union gathering in Abuja, Nigeria.

“Chad accepts the United Nations proposal to place forces on its frontier to protect the population and stabilize the sub-region,” President Déby of Chad countered during a meeting with Prime Minister de Villepin of France in the Chadian capital, N’Djamena.