The United Nations chief is urging regional and international leaders to prevent South Sudan from falling “over the abyss” into another civil war, and to speak with one voice to support a return to peace.

South Sudan's First Vice President Riek Machar is under arrest and will be investigated, the government said on Friday - the first official confirmation of a detention that world powers fear could tip the nation back into war.

United Nations Secretary-General Antonio Guterres said on Friday that a peace agreement in South Sudan was in a shambles, telling the country's leaders to put down the weapons and put all the people of South Sudan first.