A South Sudanese army general has been killed in fighting outside the rebel-held town of Bor.
A BBC correspondent with government troops said a convoy advancing on Bor came under heavy fire in an ambush.
The fighting is continuing as the warring parties meet in Ethiopia to try to agree a ceasefire. Substantive talks appear to have been delayed once again.
The conflict pits supporters of President Salva Kiir against rebels led by his sacked deputy Riek Machar.
It began on 15 December after the president accused Mr Machar of attempting a coup - which he denies.
At least 1,000 people have been killed and nearly 200,000 displaced in the conflict, which has taken on ethnic undertones. Mr Kiir is from the Dinka community and Mr Machar from the Nuer group.
The BBC's Alastair Leithead was travelling with government troops from the capital, Juba, on Sunday when the convoy came under attack about 25km (15 miles) from Bor.
The commanding general - who has not been named - was killed in the ambush.
The government has been sending reinforcements to try to retake Bor in recent days, bringing the total number of army troops involved to some 2,000.
A whole division of the South Sudanese army has joined the rebel side, so the fighting in Bor in effect involves two trained armies, our correspondent adds.
He says he saw evidence of the intensity of the fighting, with burnt-out tanks by the side of the road.
Fighting is also continuing in other areas. Army spokesman Philip Aguer said there had been clashes in the oil-producing states of Unity and Upper Nile in the north.
Up until Friday, the talks in the Ethiopian capital, Addis Ababa, were conducted by mediators. Now, teams representing the opposing factions are expected to negotiate face-to-face.
A preliminary meeting was held late on Saturday. Key issues are establishing a ceasefire, and the rebels' demand for the release of what they see as political prisoners.
But substantive talks failed to get under way on Sunday, delayed by disagreements over the agenda and - an official was quoted as saying - by "protocol issues".
It seems each side is trying to gain as much leverage on the battlefield before they even consider a ceasefire, says the BBC's Africa editor Richard Hamilton.
International mediators may be losing patience with South Sudan's leaders whose delays are costing hundreds of lives, he says.
It is now hoped talks will begin in earnest on Monday.
South Sudan spokesman Michael Makuei said the government would resist international pressure to free supporters of Mr Machar arrested in Juba at the start of the conflict.
He said releasing "those who attempted to overthrow a democratically elected government" would set a "bad precedent".
"Are we not risking the governments of Africa and the rest of the world to such attempts? We should not be arm-twisted because we are a new nation."
Meanwhile the first aid flight to South Sudan funded by the UK government has arrived in the country. The aircraft, carrying emergency aid and sanitation supplies, landed in Juba, on Sunday.
South Sudan is the world's newest state. It was formed in 2011, gaining independence from Sudan after decades of conflict.
The latest trouble has its roots in tensions that go back long before 2011.