Gamboru - Chadian forces scored a key victory over Boko Haram on Tuesday, wresting control of the border town of Gamboru within hours of launching a ground offensive against the jihadist group.
Around 2 000 Chadian troops backed by armoured vehicles crossed the border into Gamboru from the Cameroonian town of Fotokol after days of clashes with the insurgents.
By Tuesday evening, the troops had taken control of the town, according to an AFP journalist in Gamboru, leaving scenes of desolation, with houses destroyed, shops gutted and trucks charred.
The residents and remaining fighters appeared to have fled.
No official death toll from the fighting was immediately available. A Chadian military source said eight Chadian soldiers were killed and around 20 injured.
The AFP journalist saw bodies lying on the ground.
"We have routed this group of terrorists," the commander of the Chadian contingent Ahmat Dari told AFP, vowing to continue the fight against the insurgents "until the end".
The ground intervention by Chad came after the African Union last week backed a regional five-nation, 7 500-strong force to take on the extremists amid growing fears about their threat to regional security.
Nigeria has drawn fierce criticism for failing to hold back the insurgents, who have stepped up their campaign of terror in the run-up to presidential and parliamentary elections on 14 February.
Defence spokesperson Chris Olukolade denied that the presence of foreign troops on Nigerian soil compromised the country's sovereignty.
Also read: UN condemns continued attacks by Boko Haram
"Nigeria's territorial integrity remains intact," he defended, claiming national forces had "planned and are driving the present onslaught against terrorists from all fronts in Nigeria, not the Chadian forces".
Chad's intervention reflects the growing nervousness among the country's neighbours over the prospect of Boko Haram achieving its stated aim of carving out an Islamic caliphate on their borders.
The rebels have tried, in vain, to capture Maiduguri twice in the past week.
On Monday, President Goodluck Jonathan escaped a suspected suicide bomb attack after attending a campaign rally in Gombe in the northeast.
Eighteen people were reported injured in the attack believed to have been carried out by two female suicide bombers.
2 000 Chadian troops
After days of air strikes by Chadian war planes on Gamboru, Chadian forces crossed a bridge from the Cameroonian town of Fotokol to enter Gamboru.
The operation began with another barrage of air strikes lasting about an hour.
Chadian forces have also taken up position close to Boko Haram strongholds along Nigeria's border with Niger.
"A contingent of about 400 vehicles and tanks is stationed between Mamori and Bosso," Niger's private radio Anfani reported.
The massing of troops in Niger was seen as pointing the way to an imminent attack on Malam Fatori, a town held by the Islamists on the Nigerian side of a river that separates the two countries.
France is supporting the operation by carrying out reconnaissance flights over border areas of Chad and Cameroon, both former colonies, defence officials in Paris told AFP.
In a speech in the French capital, President Francois Hollande suggested that the flights also encompassed Nigeria - but defence officials insisted that the planes only went as far as the border.
The US welcomed the stepped-up role played by Chad, Niger and Cameroon in confronting the group, which gained global infamy when it abducted 276 girls from the town of Chibok in April, 2014.
Also read: Chadian troops enter Nigeria to battle Boko Haram
Lamenting the country's "troubling" handling of the crisis, Lieutenant General Vincent Stewart, head of the Defence Intelligence Agency, hailed the involvement of Chad, Cameroon and Niger as "good news".
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 sharp upsurge in violence in recent weeks is believed to be aimed at disrupting the upcoming elections.
Chad's President Idriss Deby sent soldiers to Cameroon in mid-January to assist troops from Yaounde fighting increasing rebel incursions in the country's far northeast.
N'Djamena was already part of a long-standing regional force with Niger and Nigeria in the Lake Chad area.
But that force had been assumed to be moribund after Boko Haram overran the multi-national base in Baga, northern Borno, on 3 January, in an attack that also left hundreds of civilians feared dead.
On Sunday, Nigerian troops repelled a Boko Haram assault on Maiduguri, for the second time in a week.
The city, where Boko Haram was founded in 2002 and which is currently swollen by hundreds of thousands of displaced people, is considered one of the few places where voting could feasibly take place on 14 February.