But French officials dismissed claims that the town had been captured by UFDR forces.
"This morning rebels went into Birao ... I don't know how many exactly," Christophe Prazuck, a French military spokesman, said.
But he said: "The government garrison is still in Birao, there is still a small group of French soldiers there and we're not worried."
Opposition groups first captured Birao on October 29, but government troops backed by French forces took back the town a month later.
A small contingent of French troops remained deployed in Birao to protect it.
Government officials and aid workers have said that the armed groups are still active in the area and insecurity has displaced as much as 60 per cent of the region's inhabitants.
Francois Bozize, Central African Republic's president, has accused Sudan of backing armed groups operating in his country, and says the groups have launched cross-border attacks from bases in Darfur.
Sudan denies the accusations.
Central African Republic has seen decades of army revolts, coups and rebellions since it gained independence from France in 1960.