Boko Haram militants have attacked an island on Niger's side of Lake Chad and heavy fighting ensued as the army sought to repel them, residents and security sources said.

The Lake Chad area - a vast maze of tiny islands and swampland sheltering thousands of Nigerian refugees - is thought to be serving as a hideout for the Islamist insurgent group. It was not immediately clear which island had been attacked and whether it was inhabited, but the security sources and residents said it was in Niger and within (30 miles) of the borders with Chad and Nigeria.

One of the security sources said it would provide a strategic, concealed base for the militants to co-ordinate future attacks if they won control.