Chadian troops have entered Nigeria to join the battle against militant Islamist group Boko Haram.

Armoured vehicles and infantry crossed a bridge from Cameroon following air strikes and mortar attacks on Boko Haram positions, officials say.

Fighting focused on the north-eastern town of Gamboru, Nigerian security spokesman Mike Omeri said.

Chad's deepening involvement shows how the conflict with Boko Haram is taking a regional dimension.

Last week, Chadian troops reportedly moved into Malumfatori, a Nigerian town which lies near the borders of Chad and Niger, after a ground and air assault against the militants.

'Recaptured'

Boko Haram leader Abubakar Shekau has threatened to create a caliphate, incorporating parts of Nigeria, Niger, Chad and Cameroon.

The African Union has responded by backing plans to establish a 7,500-strong regional force to fight Boko Haram.

The Chadian contingent of about 2,000 troops crossed the frontier without a shot being fired, AFP news agency reports from the scene.

Chad warplanes had earlier carried out air strikes for about an hour.

On Monday, Nigeria's army said it had recaptured Gamboru, a small town separated from Cameroon by a river.

Most of its residents had fled after Boko Haram captured it last year.

Nigeria's military has been widely criticised for failing to curb the six-year insurgency which has displaced some 1.5 million people.