(Washington): The United States has said an outbreak of violence that has left hundreds dead in South Sudan "must stop".

Anyone impeding efforts to end the fighting would be held fully accountable, the White House warned.

The UN has called for an immediate arms embargo, as well as attack helicopters to strengthen its peacekeeping force.

Violence between forces loyal to President Salva Kiir and Vice-President Riek Machar clashed in the capital city, Juba, for days.