The latest offensive against the Islamic extremists however, came at a cost as no less than 15 soldiers from both countries are feared dead.
Chad’s President, Idriss Deby, had warned that the leader of Boko Haram, Abubakar Shekau, must surrender or be killed.
At least 200 vehicles full of soldiers were spotted by residents crossing from Niger into Nigeria.
Loud detonations were soon heard, signalling heavy combat with Boko Haram, said Adam Boukarna, a resident of the border town of Bosso, Niger.
The push marks a sharp escalation by African nations against Boko Haram nearly six years after the group began its insurrection.
At an African Union summit in Addis Ababa, Ethiopia on January 31, African leaders agreed to send 7,500 troops to fight Boko Haram. Neighbouring countries later agreed to increase the force to 8,750.
UN Secretary General Ban Ki Moon has said he supports the AU’s move.
The new offensive includes troops from Niger for the first time, in addition to Chadian forces that were already carrying out missions in Nigeria, Chadian Brig. Gen. Zakaria Ngobongue said Monday.
He described extremism as a “cancer” in the region that could not be defeated by any one country alone.
“They are bandits and criminals who have nothing to do with religion,” Ngobongue said, speaking to reporters after the closing ceremony for Flintlock, an annual training exercise in counter-insurgency tactics involving some 20 countries.
About 15 soldiers from Chad and Niger died in fighting to take control of two towns in northern Nigeria from Boko Haram, the first gains against the militants in a joint offensive launched at the weekend, military sources said on Monday.
About 30 Nigerien and Chadian soldiers were wounded in the clashes over Malam Fatouri and Damasak, a day after thousands of troops crossed the border to seize areas held by the Islamist group.
A Chadian officer, who asked not to be named, said about 10 Chadian soldiers were killed and 20 wounded in fighting for the towns. Two sources with Niger’s military told Reuters five of its soldiers were killed in the clashes.
There was no official comment from the armies of either Chad or Niger.
“We have kicked the enemy out of these areas and they are now under our control,” a Niger military officer said.
The advancing troops had seized large quantities of arms and ammunition as well as vehicles, he said, and were undertaking a clean-up operation in the area. “The troops have taken dozens of Boko Haram elements prisoner,” he added.
Damasak, the town furthest into Nigeria, is 10 km (6 miles) south of the Niger border, where Nigerien and Chadian troops had been massing in recent weeks before the offensive.
A medical source in Diffa, the capital of the Niger region which borders Boko Haram’s heartland in north-eastern Nigeria, said 30 wounded soldiers had been admitted to the town’s hospital.
A second Niger military source said about 300 Boko Haram militants had been killed. There was no official confirmation of the toll and it was not possible to verify the figure.
“We had permission from Nigeria for this action,” the source said.
There was no immediate comment from Nigeria, which has launched its own offensive against the militants..