The death toll from an attack three days ago by rebel fighters in Central African Republic on a small convoy of semi-trucks filled with passengers has risen to 20, a local official said on Friday.

Twelve people were initially reported killed in Tuesday's attack, in which the rebels ambushed and set fire to the three trucks travelling to the small town of Alindao from Bambari, the seat of the war-torn Ouaka prefecture.

The authorities blamed the attack on the Coalition of Patriots for Change (CPC), an alliance of rebel groups formed ahead of last December's presidential election to oppose President Faustin-Archange Touadera.

The rebels, who are backed by former president Francois Bozize, could not be reached for comment.

"The definitive death toll is 20 deaths and six injuries," Victor Bissekoin, the prefect of Ouaka, told Reuters. "The assailants left the town but we cannot yet speak of calm."

Touadera on Thursday declared three days of national mourning for the victims.

The CPC launched a major offensive before December's election, prompting the latest violent surge in a country that has rarely known stability since independence in 1960.

Central African Republic descended into chaos in 2013 when mostly Muslim rebels ousted Bozize, sparking reprisals from mostly Christian militias.

Since then, a patchwork of rebel groups have continued to control vast swathes of territory and regularly commit abuses against the civilian population. The violence has forced about a fifth of the country's nearly 5 million people to flee their homes.