Nigeria's chief of defense staff acknowledges that the headquarters of a multinational military force on Nigeria's border with Chad has been seized by Islamic extremists.

Air Chief Marshal Alex Badeh tells reporters that only Nigerian troops were at the key base at Baga when Boko Haram attacked on Saturday because Chad and Niger had earlier withdrawn their fighters. He did not say why.

Badeh denied that Nigeria's two neighbors have withdrawn from the Multinational Joint Task Force that includes Cameroon.

Witnesses said troops fled after they ran out of ammunition.

Asked if the military would retake the base, Badeh said "Why not?" but he would not say when.

The defense chief spoke briefly to reporters after a security meeting with Nigeria's president on Tuesday.