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.

Tensions are rising in the oil-rich east African nation of South Sudan after Riek Machar who serves as a vice president was arrested in the capital.

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.