Former South Sudan Vice-President Riek Machar has said rebel troops have captured the key oil-producing state of Unity and control much of the country.
Mr. Machar also confirmed that the forces fighting the government were under his command.
The country has been in turmoil since President Salva Kiir accused Mr Machar a week ago of attempting a coup.
At least 500 people have been killed since the fighting began with the government struggling to keep control of the capital, Juba.
UN Secretary-General Ban Ki-moon called on both men to “find a political way out of this crisis”.
“I demand that all political, military and militia leaders stop hostilities and end the violence against the civilians,” he said
“I call on them to do everything in their power to ensure that their followers hear their message loud and clear.”