Boko Haram fighters from Nigeria go on a rampage in the Cameroonian border town of Fotokol, massacring civilians and torching a mosque, local residents say.

Boko Haram fighters from Nigeria have gone on a rampage in the Cameroonian border town of Fotokol, massacring civilians and torching a mosque, local residents say.

The onslaught came a day after Chad sent troops across the border to flush the jihadists out of the Nigerian town of Gamboru, which is about half a kilometre from Fotokol on the other side of a bridge.

Chad's army said it killed more than 200 militants in the intervention — the first by regional forces against Boko Haram on its home ground. Some of the insurgents, however, escaped and slipped back across the border into Fotokol at dawn to make a fresh stand.

"Boko Haram inflicted so much damage here this morning," resident Umar Babakalli said by telephone. "They have killed dozens of people." Several residents said civilians' throats were slit and that the town's main mosque was torched.

"They burnt houses and killed civilians as well as soldiers," a source close to security forces said.

Another resident who had fled to another town said he knew of at least 10 people who had been killed.

After several hours of clashes, Cameroonian troops backed by Chadian forces who scrambled back from Nigeria to help guard the town managed to repel the assault.

"People are coming back little by little to assess the damage," a source close to the Cameroonian security services said. "The survivors among the attackers have left the town." No official death toll was immediately available.

Chadian troops 'rout terrorists' in Gamboru On Tuesday, nine Chadian soldiers were killed and 21 were injured in Gamboru after about 2,000 troops backed by armoured vehicles poured across the border to take the fight to Boko Haram after days of clashes.

The sound of automatic gunfire could still be heard Wednesday in the town as the troops combed streets for remaining rebel elements.

The intervention came days after the African Union backed plans for a 7,500-strong five-nation regional force to take on the extremists, who control vast swathes of northeast Nigeria.

Nigeria's military has drawn fierce criticism for failing to rein in Boko Haram, which has stepped up its campaign of terror in the run-up to elections on February 14.

In recent months the group, which aims to establish an Islamic caliphate, has carried out increasing cross-border raids, threatening regional security.

In Gamboru, the offensive, which was preceded by days of Chadian air strikes, left scenes of desolation, with bodies lying on the ground, houses destroyed, shops gutted and trucks charred.

"We have routed this band of terrorists," the commander of the Chadian contingent Ahmat Dari said on Tuesday, vowing to "hunt them down everywhere".

Nigeria says its sovereignty 'intact' Nigeria reacted defensively to the presence of foreign troops on its soil. "Nigeria's territorial integrity remains intact," defence spokesman Chris Olukolade insisted, saying national forces had "planned and are driving the present onslaught against terrorists from all fronts in Nigeria, not the Chadian forces".

Regional forces have gone into action on several fronts. Chadian troops and vehicles massed near Boko Haram-held towns along Nigeria's border with Niger, pointing the way to another possible cross-border operation on that frontier.

France, meanwhile, is supporting the operations by carrying out reconnaissance flights over border areas of Chad and Cameroon to provide them with intelligence.

At least 13,000 people have been killed and more than a million forced from their homes since Boko Haram launched an insurgency in 2009.