President Bush's special envoy for Sudan, Andrew Natsios, has resigned, U.S. officials said Friday.

Natsios oversaw a push to end the violence that the United States calls genocide in Sudan's troubled western Darfur region and worked to maintain a 2005 peace agreement that ended decades of civil war between north and south Sudan.

The officials who confirmed Natsios' resignation spoke on condition of anonymity because they were not authorized to make the announcement, which was expected from the White House along with the nomination of his successor, former deputy U.S. Ambassador to the United Nations Richard Williamson.

Williamson, a senior Republican party official in Illinois, is close to current Deputy Secretary of State John Negroponte, whom he worked under when Negroponte was the top U.S. envoy at the U.N.

No details of Natsios' resignation were immediately available but the officials said he had been planning to step down for some time and had only expected to be in the job for about a year.

Bush appointed Natsios to the position in September 2006. From 2001 to 2005, he served as administrator of the United States Agency for International Development, the U.S. foreign aid program.