Special envoys from the United States and Nigeria are flying to South Sudan as world leaders step up efforts to avert an all-out civil war.

The US envoy for Sudan and South Sudan Donald Booth was expected in the capital Juba on Sunday.

It follows a mission by foreign ministers from east Africa and the Horn and an appeal by UN chief Ban Ki-moon to end the violence that has gripped the country.

The fighting started a week ago when President Salva Kiir accused his former deputy Riek Machar of attempting a coup.