The former vice president of South Sudan, Riek Machar, has said his troops have seized the oil-producing state Unity.
Machar also confirmed to the BBC that the forces fighting the government were under his command and now control much of the country.
The country has been in turmoil since President Salva Kiir accused Machar a week ago of attempting a coup.
According to the report, 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,' the report said.
Unity, a state on the border with Sudan, produces much of South Sudan's oil, which accounts for more than 95 percent of the country's economy.
Machar added that he was prepared to negotiate with the government if politicians arrested this week were released and transferred to a neutral country such as Ethiopia, the report added.
