Hundreds of Boko Haram fighters are attacking Cameroonian troops at a border town with Nigeria, military officers said Wednesday as Nigerian and Chadian jets pursued an offensive that has bombed the Islamic extremists out of a slew of northeastern Nigerian towns.

The attack on Fotocol, in far northern Cameroon, appears to be a response to the air attacks, Nigeria's first major offensive against Boko Haram, the Islamic extremists whose insurgency has spread across borders, prompting international concern.

The fight in Cameroon and the air offensives in Nigeria come as African Union officials meet in Cameroon to finalize a mandate for a 7,500-strong multinational force to confront the extremists who in recent months have seized more than 130 towns and villages across three of Nigeria's northeastern states bordering Cameroon, Chad and Niger. Boko Haram has held many of the towns for months, some since August.

Chadian troops joined Cameroonian soldiers in fierce fighting Wednesday against the extremists at Fotocol, according to Cameroonian military officers who spoke on condition of anonymity because they are not authorized to speak to reporters.

Earlier Nigerian jets started bombing Monday in the Sambisa Forest, where the extremists have camps and first took nearly 300 kidnapped schoolgirls last April, witnesses said.

"At night we hear distant sounds of explosions," Bulama Danbayo said by telephone from Madagali town in Adamawa state. "We were all terrified but some of the soldiers stationed here told us not to be worried, that it was soldiers that commenced bombardment of Sambisa Forest."

Chad's army said its troops were attacked Tuesday in Cameroon by Boko Haram. "Our valiant forces responded vigorously, a chase was immediately instituted all the way to their base at Gamboru and Ngala (in Nigeria), where they were completely wiped out," spokesman Col. Azem Bermendoa said on national television Tuesday night.

More than 200 extremists were killed for the loss of nine Chadian troops, he said.

Nigeria's spokesman on the insurgency, Mike Omeri, said the twin towns were recaptured but gave no credit to Chadian forces.

In a statement Tuesday, Omeri said Nigerian forces this week have "liberated from Boko Haram presence" more than a dozen northeastern towns.

The Defense Ministry spokesman, Brig. Gen. Chris Olukolade, said the presence of foreign troops on Nigerian soil in no way compromises the sovereignty of the West African nation. Nigerian forces have been unable to curb the uprising without foreign help. Nigeria is Africa's most populous nation of some 170 million people as well as the continent's biggest economy and biggest oil producer.