Cameroon’s army claimed on Tuesday to have killed more than 40 Boko Haram fighters who tried to storm a strategic border crossing from Nigeria, the same day several hundreds of Nigerian troops fled Bama, now occupied by the insurgents.

Heavily armed fighters “attempted to cross the bridge at Fotokol” in the extreme north of the country and opened fire on Cameroonian soldiers, the ministry of defence told state radio.

“Cameroon defence forces energetically reacted to this assault which lasted three hours,” the ministry said, adding that one soldier was wounded by mortar shrapnel.

There was no independent confirmation of the battle.

Gamboru Ngala, the Nigerian town on the other side of the bridge, fell to the Islamist extremists last week after they reportedly overran the Nigerian garrison there.

For several days people living in towns and villages in northeast Nigeria recently captured by Boko Haram have been fleeing towards Cameroon to escape the militants.

The extremists, who have waged a bloody insurgency for five years in northern Nigeria, seem to have changed tactics in recent months, going from spectacular kidnappings, massacres and suicide attacks to attempting to conquer territory.

Cameroon’s Defence Ministry said that 246 Nigerian soldiers and customs officials who had fled Gamboru Ngala into Cameroon to escape the Boko Haram offensive “have left the Fotokol area under military escort” to rejoin their units in Banki in Nigeria.

Several hundred Nigerian soldiers abandoned border posts further to the south along the long and isolated border last week in face of the militants’ advance, military sources said.

The Nigerian army denied its troops had fled into Cameroon, instead calling the retreat a “tactical manoeuvre”.