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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.

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

Chad's deepening involvement suggests that 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.

Boko Haram leader Abubaker Sheaku 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.

Credit: BBC