Fleeing civilians say Boko Haram Islamic extremists have overpowered a multinational military force and seized their key base on Nigeria's border with Chad.

Nigerians who escaped by canoe from Baga town to Chad say many soldiers and civilians were killed while others drowned trying to escape hundreds of insurgents firing rocket-propelled grenades and assault rifles.

They say the troops from Nigeria and neighboring Chad, Cameroon and Niger fought valiantly Saturday until they ran out of ammunition and fled, some removing their uniforms.

Baga resident Audu Labbo said Sunday night by telephone from Chad that many buildings and parts of the military base were razed.

Boko Haram has been regionalizing the conflict with recent attacks on Chadian villages and a military base.

Thousands of people have died in the five-year uprising.