Heavy fighting broke out Monday between the Chadian army and Nigeria's Boko Haram jihadists, security and local sources said.

"Violent clashes" are under way near Baga Sola, one of the main Chadian towns in the lake that straddles Chad, Cameroon, Nigeria and Niger, a Chadian security source told AFP, adding that Chadian forces had earlier "intercepted fleeing Boko Haram elements" around 20 kilometres (12 miles) to the southeast.

A source close to the local authorities said a "major search operation" was under way for Boko Haram militants hiding out on the lake's islands.