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.

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.

Mr Machar was "in confinement by the government" and his life was "at risk" after "heavily armed vehicles forcefully entered his residence", the party says.