N'DJAMENA, Chad (AP) -- Forces from Chad and Niger opened a new front in the regional military fight against the Islamic extremist group Boko Haram, as army vehicles full of soldiers crossed the border into northeast Nigeria, officials and witnesses said Monday.
The escalation in a joint military campaign against the Nigeria-based Boko Haram comes just weeks before Nigerians head to the polls for an election which many fear will turn violent, and after scores of attacks by the militants on neighboring countries who have pledged to help Nigeria defeat the extremists.
Chadian Brig. Gen. Zakaria Ngobongue said Monday that his soldiers, alongside troops from Niger, had entered Nigeria. He declined to give details about the ongoing operation. Already Chadian forces had crossed into northeastern Nigeria from Cameroon to fight the jihadis, he said.
Boko Haram has been fighting a nearly six-year insurgency against the Nigerian government, and on Saturday declared their allegiance to the extremist Islamic State group in the Middle East, raising fears that conflict could be internationalized with IS fighters from North Africa. Boko Haram has carried out mass kidnappings, including of schoolgirls.
"They are bandits and criminals who have nothing to do with religion," Ngobongue said. He spoke to reporters after the closing ceremony for Flintlock, a training in counter-terrorism tactics that included U.S. special forces and involved 20 countries.
Witnesses in the Niger town of Bosso reported about 200 military vehicles crossing over into Nigeria since Saturday. Adam Boukarna, one resident, said the deployment was followed by loud detonations, signaling heavy combat with Boko Haram.
Describing the stepped-up military activity, Nigerian military spokesman Brig. Gen. Chris Olukolade said Sunday night that "there were some pre-emptive manoeuvers along an axis in the theater. Nigerian forces were also involved."
Cameroon's minister of defense, Edgard Alain Mebe Ngo'o, said troops from Nigeria and Chad would fight Boko Haram while soldiers from Cameroon and Niger would guard their borders to prevent the militants from escaping. Boko Haram has been using Cameroon as an escape and supply route.
Residents in potential conflict zones in Cameroon have been asked to leave, the minister said.
Tens of thousands of Nigerians have taken refuge in the area, among 1.6 million people driven from their homes by the insurgency.
International concern has increased along with the casualties. About 10,000 people were killed in Boko Haram's uprising last year, compared to about 2,000 in the four previous years, according to the U.S. Council on Foreign Relations.
The United States, Britain, France and the European Union are backing the formation of a multinational force of 8,750 troops led by Nigeria and Chad with contingents from Cameroon, Niger and Benin. Several other countries also have pledged to help.
On Saturday, Boko Haram leader Abubakar Shekau reportedly pledged his group's allegiance to the Islamic State group, raising fears that the conflict largely restricted to northeast Nigeria and its neighbors' borders could be internationalized.