Chadian troops took control of the Nigerian border town of Gamboru on Tuesday after launching a ground offensive against the jihadist Boko Haram group, an AFP journalist reported.

The Chadian army crossed into Gamboru, which Boko Haram militants had seized several months ago, from the Cameroonian town of Fotokol after days of intense fighting.

Chad had sent ground troops into Nigeria for the first time on Tuesday to fight Boko Haram in a sign of increased regional pressure on the Islamist rebels after weeks of surging violence.

The Chadian offensive on the town of Gamboru, which borders Cameroon, follows days of clashes with the Islamists who control vast swathes of northeast Nigeria.

The action comes 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 February 14.

Defence spokesman Chris Olukolade denied that the presence of foreign troops on Nigerian soil compromised the country’s sovereignty.

“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 ground intervention reflects the growing nervousness among Nigeria’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 the strategic northeastern Nigerian town of Maiduguri twice in the past week.

On Monday, President Goodluck Jonathan — who is running for re-election against a former military ruler who has vowed to defeat Boko Haram — 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 be carried out by two female suicide bombers.

After days of airstrikes by Chadian war planes on Gamboru, Chadian armoured vehicles Tuesday rolled into the town across a bridge from the Cameroonian town of Fotokol, followed by a contingent of around 2,000 troops.

No shots were fired as they crossed the frontier after another barrage of airstrikes lasting about an hour, an AFP correspondent in Fotokol witnessed. 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 missions only went as far as the border.

Boko Haram, which launched its bloody insurgency in 2009, has seized dozens of towns and villages in the last six months. The sharp upsurge in violence in recent weeks is believed to be aimed at disrupting this month’s 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 January 3, in an attack that also left hundreds of civilians feared dead.

– ‘Snipers everywhere’ – The operation in Gamboru comes hours after Nigerian authorities said Monday the town had been reclaimed after three days of air strikes by Chadian warplanes. The government also claimed other military successes against the group.

A Chadian army officer told AFP however the insurgents in Gamboru were “in the whole town, hiding in the houses, and they have posted snipers everywhere”. On Sunday, Nigerian troops repelled a Boko Haram assault on the Borno state capital, 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 refugees from the violence, is considered one of the few places where voting could feasibly take place on February 14.