Sudan's Foreign Ministry, however, denied that anyone who spoke to council members during their October 5-9 visit to Sudan had been arrested.

The U.S. official, who spoke on condition of anonymity, said the United States has been urging the Sudanese government in Khartoum and its diplomats at the United Nations to release the Darfuris and cease the harassment of those who talked to council envoys on their visit.

"It's obviously unacceptable," the official said, adding that the United States has asked U.N. officials to brief the 15-nation Security Council on the matter on Monday. He said that U.S. Ambassador to the U.N. Susan Rice was personally involved in efforts to try to free the detained Darfuris.

The U.S. official said Washington first heard about the arrests a week ago.

The U.S. and British envoys on the Security Council trip to Sudan told reporters refugees had complained to them directly about hunger and deteriorating security in the arid region.