The State Department said that Stuart Symington, a retired ambassador with long experience in Africa, "will lead US efforts to support the peace process and a successful political transition in South Sudan."

The United States has been a major supporter of the impoverished, majority-Christian nation that won independence from Sudan in 2011, contributing around $1 billion a year in food and other humanitarian aid.

A fallout soon after independence between President Salva Kiir and rebel leader Riek Machar has triggered brutal violence that has left almost 400,000 people dead and displaced four million more.

African mediators have twice given the two leaders extensions in an agreement to form a unity government, with the latest deadline looming in mid-February.

The United States has voiced anger at the bickering duo's failure to resolve differences and has imposed sanctions on two sitting ministers, accusing them of obstructing peace efforts.