"We are evaluating the situation with Central African forces to determine how to proceed," he said.
Kader Kennengad, a lieutenant with the UFDR, was reported by AFP as saying that Birao had fallen into rebel hands after "fierce fighting".
One military source, who asked not to be identified, confirmed an attack on the town near the borders with Chad and Sudan, but could not confirm who was responsible or whether Birao had been captured.
Northeastern rebels first captured Birao on October 29, but French forces backing government troops took back the town a month later, attacking with jet fighters and helicopter gunships.
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.