Rebel forces have taken over a city in the southeastern Central African Republic, an official with the United Nations mission in the country said.
The rebels carried out an attack at dawn and are now everywhere in Bangassou, the official said, adding that fighting was continuing in several parts.
A rebel coalition has attacked several cities in recent weeks and currently controls two-thirds of the country.
They accuse the government of holding a fraudulent election on December 27.
The government denied allegations of electoral fraud. The initial results of the election for president and seats in the National Assembly are expected on Monday.
The authorities also accused former president François Bozizé, who supports the rebels, of trying to deliver a coup – something he denied.
The Central African Republic (CAR) is a resource-rich but deeply unstable country that has suffered several blows since France’s independence 60 years ago.
BBC Africa editor Mary Harper says there appears to be no decrease in rebel activity.
On Saturday, the rebel coalition attacked the city of Damara, which is about 70 km north of the capital Bangui.
CAR is one of the poorest countries in Africa, although it is rich in resources like diamonds and uranium. The UN, which has about 13,000 peacekeepers in place, estimates that half the population depends on humanitarian assistance.