Boko Haram militants have been involved in heavy fighting.

Nigeria's military says its troops have retaken a major border town and killed scores of extremist fighters, although witnesses also report deadly Boko Haram attacks on other villages.

Meanwhile, a military official in Niger says seven soldiers were killed in an overnight attack on an island in Lake Chad, which touches the four countries targeted by the militants. The official says 14 Boko Haram assailants were also killed in the fighting.

A Nigerian Defense Ministry spokesman says troops won back the garrison town of Baga, on Lake Chad and the border with Cameroon, after a fighter jet bombarded the area and soldiers advanced on booby-trapped roads by dismantling some 1,500 land mines.

The spokesman says many insurgents drowned in trying to escape as soldiers stormed into Baga early Friday.