Nigerian Boko Haram fighters went on a rampage in the Cameroonian border town of Fotokol on Wednesday, massacring dozens of civilians and torching a mosque before being repelled by regional forces.
The onslaught came a day after Chad sent troops across the border to flush the armed men out of the Nigerian town of Gamboru, which lies some 500 yards from Fotokol on the other side of a bridge.
Chad's army said it had killed more than 200 Boko Haram fighters in the intervention — the first by regional forces against Boko Haram on its home ground. But some of the fighters escaped, it added.
On the Cameroonian side of the border, the Boko Haram assault on Fotokol left nearly 70 civilians and six Cameroonian soldiers dead, a local security source told Agence France-Presse (AFP).
There were also Boko Haram bodies "everywhere," the source added.
"Boko Haram inflicted so much damage here this morning. They have killed dozens of people," Umar Babakalli, a resident of Fotokol, told AFP by telephone.
Several residents said civilians' throats were slit and that the town's main mosque was torched.
"They burnt houses and killed civilians as well as soldiers," a source close to security forces said.
After several hours of clashes, Cameroonian troops, backed by Chadian forces who scrambled back from Nigeria to help guard the town, managed to repel the assault.
No official death toll was immediately available.
On Tuesday, nine Chadian soldiers were killed and 21 were injured in Gamboru after around 2,000 troops backed by armored vehicles poured across the border to take the fight to Boko Haram after days of clashes.
The sound of automatic gunfire could still be heard in the town later Wednesday as troops sought remaining rebel elements.
The intervention came days after the African Union backed plans for a 7,500-strong five-nation regional force to take on Boko Haram, which controls vast swathes of northeast Nigeria.
'Hunt them everywhere'
Nigeria's military has drawn fierce criticism for failing to rein in Boko Haram fighters, who have stepped up their violent campaign in the northeast in the run-up to presidential and parliamentary elections on Feb. 14.
In recent months, Boko Haram, which aims to establish an Islamic caliphate, has also carried out increasing cross-border raids, threatening regional security.
Viewing the widening field of Boko Haram activity a direct threat to its national interests, Chad has deployed its war-tested army to join the fight against the group, and has reportedly now entered Nigeria in at least two places.
According to several sources, Chad has also amassed forces and hundreds of vehicles along the border area between Nigeria and Niger -- a zone not far from Boko Haram's stronghold.
N'Djamena has not yet officially confirmed its troop movement into Niger, but it is now thought Chad may be positioning its forces to be able to trap Boko Haram in pincer offensives launched simultaneously from the north and south.
In Gamboru, the offensive, which was preceded by days of Chadian air strikes, left scenes of desolation, with bodies lying on the ground, houses destroyed, shops gutted and trucks charred.
"We have routed this band of terrorists," the commander of the Chadian contingent Ahmat Dari told AFP Tuesday, vowing to "hunt them down everywhere."
Nigerian sovereignty 'intact'
Nigeria has reacted defensively to the presence of foreign troops on its soil.
"Nigeria's territorial integrity remains intact," defense spokesman Chris Olukolade insisted, claiming national forces had "planned and are driving the present onslaught against terrorists from all fronts in Nigeria, not the Chadian forces."
France is also supporting the operations by carrying out reconnaissance flights over border areas of Chad and Cameroon to provide them with intelligence, defense officials in Paris said.
At least 13,000 people have been killed and more than a million forced from their homes since Boko Haram launched an insurgency in 2009.
The group has stepped up its attacks in recent weeks, in a move believed to be aimed at disrupting elections.
The rebels have tried, in vain, to capture the strategic northeastern town of Maiduguri twice in the past week.
In January, Boko Haram fighters carried out a massacre in a town on Lake Chad that houses a regional military base.
Hundreds of civilians were reported killed in the attack, according to Amnesty International.