KANO, Nigeria - A suicide bomber blew himself up on Sunday inside a church in the restive northeastern Nigerian city of Potiskum, killing five worshippers in the latest in a series of attacks, a police officer and witness told AFP.

The bomber entered the Redeemed Christian Church of God at 9:55 am in the Jigawa area on the outskirts of the city and detonated his explosives. “Four worshippers died instantly while the fifth victim gave up shortly after she was taken to hospital,” a police officer involved in the evacuation told AFP. “The victims included a woman and her two children, the pastor and another worshipper,” added the officer who spoke on condition of anonymity.

Witness Garba Manu told AFP that the bomber arrived on a motorised rickshaw, adding that “as soon as he entered a loud explosion ripped through the church which is under construction.”

“I saw him walking in and he didn’t raise any suspicion,” Manu said. “The dismembered body of the bomber was also recovered.” There was no immediate claim of responsibility but Potiskum, the commercial capital of Yobe state, has been hit repeatedly by Boko Haram Islamists in their six-year insurgency, including by suicide bombers.

The insurgents have also launched a series of attacks on mosques, villages and markets in neighbouring Borno state in the past few days, claiming more than 200 lives. Boko Haram are keen to prove they are not a spent force despite a four-nation military offensive that has run them out of captured towns and villages since February this year. The use of improvised explosive devices and suicide bombings has increased since May 29 when President Muhammadu Buhari took office vowing to crush the rebellion. According to an AFP tally, at least 450 people have lost their lives since that date.

The army said on Sunday its troops must have killed more than 600 insurgents in the restive northeast in the past one month. “Over 600 terrorists must have been killed in the last one month while other insurgents are finding life extremely difficult,” the army said in a statement. It said despite “guerrilla tactics of using vulnerable girls and young men for suicide attacks on soft targets, we ensure that their fighters do not escape as they continue to meet their Waterloos in the hand of the troops.”

The army said it had also ensured that the Islamists had not captured any territory since Buhari came to power.

“Since the emergence of new administration of President Muhammadu Buhari no single territory in Nigeria is being occupied or proclaimed by the terrorists as their ‘Caliphate’ even as their leaders are either being killed, captured or on the run,” it added.

Buhari has condemned the recent attacks as “inhuman and barbaric” and again vowed to end the Islamists’ six-year-old insurgency which has killed at least 15,000 people and displaced 1.5 million others.

The spike in violence has sparked concern that earlier gains by the armies of Nigeria, Niger, Chad and Cameroon against the militants were being eroded.

The four countries - all of which border Lake Chad, a focal point of Boko Haram unrest - took the fight to the militants early this year to try claw back some of the territory they had gained in the northeast.