Natsios had overseen a push to end violence, which the US calls genocide, in the western Darfur region.
 
He also worked to maintain a 2005 peace agreement that ended decades of civil war between north and south Sudan.
 
Resignation
 
After news of the resignation became public, Darfur advocates called for the White House to appoint a full-time envoy to deal with the crisis which has killed more than 200,000 people and displaced more than 2.5 million since 2003.
 
"The president should appoint a full-time envoy answering directly to him, and end the crippling turf battles once and for all," said John Prendergast, co-chair of the ENOUGH project and a board member of the Save Darfur Coalition.
 
Prendergast said Williamson, Natsios's replacement, is a "hard-nosed negotiator" who cares deeply about the plight of the Sudanese people and might be more able than Natsios to break through the bureaucracy to make a mark on Sudan policy.
 
Williamson is a senior Republican party official who has in the past worked closely with John Negroponte, now Deputy Secretary of State.
 
Natsios plans to return to a full time teaching position at Georgetown University.