A girl thought to be as young as seven killed herself and five others in a suicide bombing in north-east Nigeria on Sunday as the country’s president, Goodluck Jonathan, conceded that his government had underrated the capacity of the Islamist extremist group Boko Haram.

The attack on a market in the town of Potiskum is the latest in a string of suicide strikes in which children have been used. Previous attacks have been blamed on Boko Haram.

Nineteen people were injured in the blast in Yobe state’s commercial capital, a local vigilante leader said. “So far, five people were killed with the girl while 19 others have been taken to hospital for injuries,” Buba Lawan said.

A hospital source speaking on condition of anonymity confirmed the toll.

The bombing highlights the severe security challenges facing Nigeria in the runup to presidential and parliamentary elections on 28 March.

During a trip through neighbouring Chad, Cameroon and Niger, the French foreign minister, Laurent Fabius, on Sunday urged Nigeria to commit itself fully to battling Boko Haram.

“It is necessary that there be full commitment from Nigeria in the fight against Boko Haram,” he told reporters during a press conference in Niger’s capital, Niamey.

On Saturday in the Chadian capital N’Djamena, Fabius visited a coordination cell set up at a French military base between Cameroon, Chad, Niger and France.

Paris has promised to increase intelligence-sharing and other assistance to the armies of Nigeria and its three neighbours, which banded together to battle Boko Haram after the extremists expanded their campaign across the region’s borders.

Jonathan, who has been in office since 2011, is in a tough re-election campaign against the former military ruler Muhammadu Buhari.

Voting initially scheduled for 14 February has been delayed for six weeks to give Nigeria’s military time to secure the country, despite its failure to beat Boko Haram back over the past six years.

Jonathan admitted in an interview published on Sunday that he hadunderestimated the Islamists, who have overrun swaths of the north-east.

“Probably at the beginning, we, and I mean myself and the team, we underrated the capacity of Boko Haram,” Jonathan told the newspaper ThisDay.

He said the military has recently acquired more arms and ammunition, and vowed that Boko Haram’s defeat was imminent.

Sunday’s bombing in Potiskum was the second suicide attack in or near the market where new and secondhand phones are sold and repaired.

The first attack occurred on 11 January, when two suicide bombers - one of whom appeared to be around 15 - blew themselves up outside the market killing six people and injuring 37 others.

Before Sunday’s strike, suspicious security guards and vigilantes said they tried to prevent the girl, whom witnesses said appeared around seven, from entering the market.

“We sent her back four times, because given her age, she did not have anything to do in the market,” Lawan said.

“When we were screening people, she bent and tried to pass under the ropes, some distance from our view. That was when the explosives went off.”

In a sign of the distrust generated by the suicide campaign, Lawan said “we have barred women from entering the market to prevent further attacks”.

More than 13,000 people have been killed and more than a million people left homeless since 2009 as the rebels try to carve out an Islamic state.