The U.S. special envoy for South Sudan is calling for a "reconstruction" of the power-sharing agreement in the country's peace deal, just a year after the United States pressured both sides to sign it.

Donald Booth told Congress on Wednesday that "we do not believe it would be wise" for rebel leader Riek Machar to return to his role as vice president after he fled the country amid renewed fighting in July.

Neighboring Sudan has said Machar is now in that country. Machar's rival, President Salva Kiir, has replaced him as vice president.

South Sudan's peace deal was signed in August 2015 to end a civil war that began in December 2013 and killed tens of thousands in the world's youngest country. Hundreds were killed in the July fighting.