Terrorists belonging to the Boko Haram group active in Nigeria killed at least 26 people in an attack in neighboring Cameroon that was one of the bloodiest in recent months, according to a security official.

"Seventeen soldiers and nine civilians" were killed in the attack early Monday on Darak, a Cameroonian island located near Lake Chad, the regional security official told AFP.

A preliminary count, given on Monday, had three Cameroonian soldiers killed along with an unspecified number of civilians in an attack on a military post in the area.

The official told AFP on Tuesday that the Boko Haram attack sparked fierce combat with Cameroonian troops. He said 40 of the terrorists were captured and seven soldiers remained missing.

Before being repelled, the terrorists managed to hoist their flag over Darak, several security sources said.

Boko Haram's decade-long uprising in Nigeria's northeast, which has killed more than 27,000 people and left 1.8 million homeless, has spilled into neighboring Niger, Chad and Cameroon.

An anti-Boko Haram force combining soldiers from Chad, Cameroon, Niger, and Nigeria has been set up but has failed to rout the group from the restive Lake Chad region.