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 the militants have attacked 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 jihadists, he said.

Boko Haram has been fighting a nearly 6-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 the 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 maneuvers along an axis in the theater. Nigerian forces were also involved.”

Military spokesmen from Nigeria and Niger could not immediately be reached for further comment Monday.

Report from The Associated Press