The Sudan Liberation Army (SLA) attacked Golo, a town in the central Jabal Marra region of Darfur that has changed hands several times in the three-year conflict, the sources and a US official said on Tuesday.
Without giving details, a senior African Union official said: "The SLA launched an offensive on Golo yesterday. The government reacted."
The AU has about 7000 troops in Darfur monitoring a ceasefire that has regularly been violated by both sides.
An aid worker in Darfur in close contact with the region around Golo also said the SLA had launched an offensive on the town and said several people were injured.
Jendayi Frazer, the US assistant secretary of state for Africa, said about six soldiers were killed in the attack, in an area controlled by Abd al-Wahid Muhammad al-Nur, an SLA leader.
Golo has been a focus for tension as it is now in government hands, but overlooked by hills which are a rebel stronghold.
"This is bad and ... it points towards a splintering of the rebel movements," she said.
"It suggests we really need to speed up the talks - it's a very fragile situation"
US assistant secretary of state for Africa
"It suggests we really need to speed up the talks - it's a very fragile situation."
The SLA and another Darfur rebel group had said late on Monday they had suspended Darfur peace talks to protest against Sudan's candidacy as head of the African Union, which is sponsoring the negotiations in the Nigerian capital Abuja.
But at least one group said on Tuesday it would resume talks as the AU did not choose Sudan as its head, opting instead for Congo Republic's president, Denis Sassou Nguesso.
Critics had said a Sudanese presidency would damage the AU's image.
Sudan's Foreign Ministry said it would take over the AU chairmanship in 2007 after Congo's term ends.
Rebels launched their uprising in 2003 complaining that the government was neglecting their region.
About 2 million people have been made homeless in the fighting.