The militant group Boko Haram has seized a town and key multinational military base in north-eastern Nigeria, officials and eyewitnesses say.

A senator in Borno state said troops had abandoned the base in the town of Baga after it was attacked on Saturday.

Residents of Baga, who fled by boat to neighbouring Chad, said many people had been killed and the town set ablaze.

Baga, scene of a Nigerian army massacre in 2013, was the last town in the Borno North area under government control