South Sudan troops trying to recapture the oil hub of Bentiu from rebels have been forced back amid heavy gunfire, a BBC correspondent says.
Government troops advanced towards the town centre over the weekend with an armoured column.
But the BBC's Alastair Leithead, who is in the UN compound on the outskirts, saw a contingent of troops in retreat.
The town has changed hands several times since fighting broke out in South Sudan last December.
Tensions came to a head after President Salva Kiir accused his sacked deputy, Riek Machar, of plotting a coup.
Rebel forces deny UN charges that they killed hundreds of people along ethnic lines after seizing Bentiu in April.
Government forces moved into the town, the capital of Unity State, on Sunday.
But it appears at least a section of the troops have been forced to retreat, our correspondent says.
He saw government soldiers withdrawing past the UN compound where he is based on Monday.
It is unclear which side currently has control of the town.
The fighting comes after the US Secretary of State John Kerry said on Friday he had secured an agreement from President Kiir and Mr Machar for peace talks to be held in Addis Ababa.
The power struggle between the two men - who fought together in the civil war before South Sudan's independence - has increasingly taken on an ethnic dimension.
Mr Kiir is a member of the country's largest group, the Dinka, while Mr Machar is from the second-biggest, the Nuer.
The conflict has displaced about a million people, and both sides have been implicated in atrocities and war crimes.
The UN has about 8,500 peacekeepers in South Sudan, which became the world newest state after seceding from Sudan in 2011.