Forces from Chad and Niger have 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 north-east Nigeria.
The escalation in Africa's joint battle against the Nigeria-based jihadists 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 neighbouring countries who have pledged to help Nigeria defeat the extremists.
Chadian Brig Gen Zakaria Ngobongue said his soldiers, alongside troops from Niger, had entered Nigeria but he declined to give details about the ongoing operation. Chadian forces had already crossed into north-eastern Nigeria from Cameroon to fight the jihadis, he said.
Boko Haram has been fighting a six-year insurgency against the Nigerian government, and on Saturday declared their allegiance to the extremist Islamic State group in the Middle East. Boko Haram has carried out mass kidnappings, including of schoolgirls.
"They are bandits and criminals who have nothing to do with religion," Mr Ngobongue said. He spoke to reporters after the closing ceremony for Flintlock, a military training event that included US special forces and involved 20 countries.
Witnesses in the Niger town of Bosso reported about 200 military vehicles crossing over into Nigeria. Adam Boukarna, one resident, said the deployment was followed by loud detonations, signalling heavy combat with Boko Haram.
Describing the stepped-up military activity, Nigerian military spokesman Brig Gen Chris Olukolade said last night that "there were some pre-emptive manoeuvres along an axis in the theatre. Nigerian forces were also involved."
Cameroon's minister of defence 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.