Thousands of people have died and more than a million fled their homes in central African country.
The ceasefire was signed in Brazzaville after three days of talks hosted by President Denis Sassou N’Guesso.
Doctors without borders says almost 3,000 people have been killed in the CAR.
Doctors Without Borders says the death toll is nearing 3,000.
The attack in Bambari comes a day before French Minister Jean-Yves Le Drian was due to visit.
AU official said the organisation is making progress towards a regional military force by end of 2015.