Rebel forces in the Central African Republic (CAR) have captured the key northern town of Ndele following a surprise attack, an army source has told the BBC.
The rebels said they had also seized two smaller towns, Sam Ouandja and Ouadda, but this could not be confirmed.
Ndele is on a major route linking the CAR to Sudan, Cameroon and Chad.
The CAR has had a series of rebellions and coups since independence in 1960.
It is rich in mineral resources, including gold and diamonds, but its population is extremely poor.
The latest conflict involves a faction of the Union of Democratic Forces for Unity (UFDR), which accuses President Francois Bozize's government of reneging on a peace accord signed in 2007.
The accord led to rebel forces being integrated into the army.
But some of the rebels have since deserted and taken up arms again.
An army source, who spoke on condition of anonymity, told the BBC that the rebels captured Ndele after a surprise attack.
The town was poorly defended, as a detachment of troops was leaving Ndele and had not yet been relieved by other soldiers, the source said.
The violence has forced many residents to the town, which has a population of between 15,000 and 20,000, AFP news agency reports.
The army in Ndele was backed by a former rebel movement, the Convention of Patriots for Justice and Peace (CPJP), which signed an accord with the government in 2011, AFP reports.
"The CPJP put up resistance, but they were routed by our men and forced to flee," a rebel spokesman known as Col Narkoyo told AFP.
He said the northern towns of Ouadda and Sam Ouandja were also under rebel control.
"Our forces took prisoners among FACA [army] elements at Sam Ouandja," Col Narokoyo is quoted as saying.
The BBC French Service's Ibrahima Diane says the UFDR faction has carried out several attacks in recent months, but this was its biggest offensive.
The fall of Ndele is a blow to the government, he says.