US envoy to Sudan, South Sudan stepping down - Reuters

Lyman's departure comes after a nearly two-year stint as Obama's troubleshooter for the region, a tenure that saw South Sudan secede last year under a U.S.-backed peace deal that ended decades of civil war.

But unresolved disputes between the two countries, including renewed border hostilities, have impeded normalization of ties.

"Princeton has done a tremendous job in helping to realize the promise of an independent South Sudan, and working toward the international vision of Sudan and South Sudan living side by side in peace," Obama said in statement.

"The people of Sudan and South Sudan, who have suffered so much, have the opportunity to seize a brighter future because of Princeton's efforts to urge both sides to put the interests of their people first."

The Obama administration did not provide any immediate explanation for the departure of Lyman, 77, who was named to the post in March 2011.

Before that, he led a U.S. team that supported negotiations for Sudan's 2005 peace agreement, and also served previously as deputy assistant secretary of state for African affairs, U.S. ambassador to Nigeria and South Africa as well as assistant secretary of state for international organization affairs.

Sudan and South Sudan have asked the African Union to help hammer out details of a proposed withdrawal of troops from their disputed border, defense ministers from both countries said on Monday, as stalled talks continued to delay oil exports from landlocked South Sudan through Sudan.

(Reporting By Matt Spetalnick; editing by Christopher Wilson)