Riek Machar said rebels have captured the key oil-producing state of Unity, according to BBC.

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 in what many now see describe as a civil war which 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."

South Sudan gained independence in July 2011 after its people overwhelmingly voted in a referendum for a split from the North.

The government in Juba is grappling with rampant corruption, unrest and conflict in the deeply impoverished but oil-rich nation, left devastated by decades of war.

SHI/SHI