Enrico Carisch, a Swiss national who until October led a U.N. panel investigating violations of the arms embargo, contrasted the administration's efforts with those of President George W. Bush, noting the previous administration's strong advocacy of sanctions against Sudan.

"In contrast to that leadership of 2004 and 2005, the United States appears to have now joined the group of influential states who sit by quietly and do nothing to ensure that sanctions work to protect Darfurians," Carisch said in written testimony for an appearance before the House Foreign Affairs subcommittee on Africa.

Carisch said key architects of the U.N. arms embargo -- the United States, France and Britain -- have lessened their commitment to enforcing sanctions as U.S.-led efforts to revive peace talks in Sudan have gained traction. "Increasingly it looks like poorly understood and under-enforced U.N. sanctions are being sold out in favor of mediation whose success is far from ensured," he said.

The U.N. Security Council, he noted, has dismissed nearly 100 recommendations in recent years aimed at strengthening the sanctions, including proposals to extend them to all of Sudan. He also said U.S., French and British officials have done little to press for an official debate on the arms embargo in the Security Council.

Obama administration officials challenged Carisch's characterization, saying that Susan E. Rice, the U.S. ambassador to the United Nations, has been a passionate proponent of tough sanctions and recently implored the world body to provide a more candid account of the Sudanese government's misbehavior in Darfur.

"The Obama Administration is actively engaged in ensuring enforcement of all UN sanctions regimes. Given the priority that this Administration attaches to Sudan -- and Ambassador Rice's well-known hard-line views on the issue, it is not credible to say that U.S. efforts have been anything less than vigorous," Rice's spokesman, Mark Kornblau, said in an e-mail. "The United States is the most active member of the Security Council in pushing for better enforcement of sanctions and action to protect civilians in Darfur even in the face of a divided Security Council."

Carisch alleged that large amounts of foreign ammunition and weapons, principally from China and Chad, have illegally made their way into Darfur in recent years, fueling a conflict that has left more than 300,000 dead and driven more than 2.7 million from their homes.

Also at the hearing, the top U.S. envoy for Sudan, retired Air Force Maj. Gen. J. Scott Gration, provided his first briefing to Congress since the Obama administration announced its strategy for achieving peace in Sudan.

Gration said that top U.S. officials would meet next month to assess Sudan's support for U.S. goals there and to determine whether to reward Khartoum or impose more sanctions. The goals include political settlements in Darfur and southern Sudan and assurances that Sudan will not provide a haven to terrorists.