Rebel forces have seized a southeastern city in the Central African Republic, said a United Nations mission.
The rebels carried out a dawn attack and were now all over Bangassou, the official said, adding that fighting was continuing in various parts.
A rebel coalition has attacked several towns over the past few weeks and is currently taking control of two-thirds of the country.
They accuse the government of holding a fraudulent election on December 27.
The government has denied the allegations. The initial results of the presidential election and seats in the national assembly are expected on Monday.
Authorities also accused former president François Bozizé, who supports the rebels, of trying to stage a coup – something he denied.
The Central African Republic (CAR) is a rich but deeply unstable country that has seen several coups since the independence of France 60 years ago.
Mary Harper, editor of BBC Africa, says there seems to be no disappointment in rebel activities.
On Saturday, the rebel coalition attacked the city of Damara, which is about 70 km north of the capital Bangui.
The CAR is one of the poorest countries in Africa, although it is rich in resources such as diamonds and uranium. The UN, which has nearly 13,000 peacekeepers on the ground, estimates that half of the population is dependent on humanitarian aid.