Civilians have been fleeing the fighting in Jonglei state
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 to the BBC 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.
The BBC's former Sudan correspondent James Copnall says the situation now looks very much like civil war.
At least 500 people have been killed since the fighting began.
Mr Machar's comments came as four US service personnel involved in evacuating US citizens were wounded when their aircraft were shot at in Bor, the capital of eastern Jonglei state.
The three CV-22 Ospreys were attacked as they approached Bor, which is occupied by forces loyal to Mr Machar, a US military statement said.
All four wounded were treated in Nairobi, Kenya, and were in a stable condition, it added.
Our correspondent says Mr Machar's forces were likely to have fired on the aircraft thinking they were Ugandan.
Oil state
Mr Machar told the BBC that a senior military commander, Gen James Koang, had followed Gen Peter Gadet, who had admitted defecting to the rebels earlier in the week.
"James Koang is now military governor of Unity state. He is just like the military governor in Jonglei - Peter Gadet," he said.
However, government forces say Gen Koang defected alone and did not take any troops with him.
Unity, a state on the border with Sudan, produces much of South Sudan's oil.
Oil accounts for more than 95% of the country's economy.
Mr Machar added that he was prepared to negotiate with the government if politicians arrested earlier this week after the coup attempt was announced were released.
"I accept talks, if the detained political leaders are released, and deported to neutral ground, for example Addis Ababa. These leaders will be participating in talks," he said.
Mr Kiir also agreed to negotiations after meeting African mediators on Friday.
But government troops are currently trying to retake Bor, in one of the most volatile regions in the country.
Troops backed by helicopter gunships were advancing on the town, army spokesman Philip Aguer told AFP news agency.
A spokesman for UN peacekeepers Unmiss in Bor said considerable numbers of people had arrived over the past 24 hours from surrounding areas seeking their protection.
Evacuations
Meanwhile Uganda has denied reports that its aircraft are bombing the town.
Uganda is one of a number of countries trying to evacuate their citizens, and has sent troops to take part in the operation. They will also try to secure the capital Juba, just 75km (50 miles) from the Ugandan border, reports say.
President Kiir, a member of the majority Dinka ethnic group, sacked Mr Machar, who is from the Nuer community, in July.
He said that last Sunday night uniformed personnel opened fire at a meeting of the governing party, the Sudan People's Liberation Movement (SPLM).
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Violence then broke out in Juba and has since spread across the country, pitting gangs of Nuer and Dinka against each other.
Thousands of civilians have flocked to UN compounds seeking shelter from the unrest.
Two Indian peacekeepers and at least 11 civilians were killed in an attack on a UN compound in Akobo, Jonglei state, on Thursday.
Mr Machar's whereabouts are unknown, though he is thought to be inside the country.
Jonglei state has seen some of the worst violence since South Sudan became independent from Sudan in 2011, with hundreds killed in periodic clashes between rival heavily-armed ethnic militias sparked by cattle-rustling.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north.
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