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.
Machar also confirmed that the forces fighting the government were under his command.
The country has been in turmoil since President Salva Kiir accused 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.
US President Barack Obama has warned that any effort to seize power through the use of military force will result in the end of longstanding support from the United States and the international community.