The attack on a market in the city of Potiskum, the commercial capital of Yobe state, was the latest in a string of suicide strikes in which children have been used.
The initial death toll given by witnesses and hospital sources was six — the bomber and five others — but medical sources at the state-run hospital in Potiskum said later two of those injured had also died.
Previous attacks have been blamed on Boko Haram.
Nineteen people injured in the blast were taken to the hospital, a local vigilante leader, Buba Lawan, told AFP.
The bombing highlights the severe security challenges facing Nigeria in the run-up to presidential and parliamentary elections on March 28.
During a swing through neighbouring Chad, Cameroon and Niger, French Foreign Minister Laurent Fabius on Sunday urged Nigeria to entirely commit itself to battling Boko Haram.
“It is necessary that there be full commitment from Nigeria in the fight against Boko Haram,” Fabius told reporters in Niger’s capital, Niamey.
On Saturday in the Chadian capital N’Djamena, Fabius visited a coordination cell set up on a French military base to liaise 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.
– ‘Underrated Boko Haram’ –
President Goodluck Jonathan, who has been in office since 2011, is engaged in a tough re-election campaign against ex-military ruler Muhammadu Buhari.
Voting initially scheduled for February 14 has been delayed for six weeks to give Nigeria’s military time to secure the country, despite its failure to beat back Boko Haram in the previous six years.
Jonathan admitted in an interview published on Sunday that early on in their rise he had underestimated the Islamists, who have overrun swathes of the northeast.
“Probably at the beginning, we, and I mean myself and the team, we underrated the capacity of Boko Haram,” Jonathan told newspaper ThisDay.
He said the military had recently acquired more arms and ammunition to battle the Islamists, and vowed their defeat was imminent.
Sunday’s bombing in Potiskum was the second suicide attack in or near the market where new and second-hand phones are sold and repaired.
The first attack occurred January 11, when two suicide bombers — one of whom appeared to be aged 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 — who 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 bombings, Lawan said “we have barred women from entering the market to prevent further attacks.”
Over 13,000 have been killed while more than one million people have been left homeless since 2009 as the Boko Haram militants try to carve out an Islamic state in Nigeria’s northeast.
Nigeria’s military said Sunday that the offensive against Boko Haram was making progress, claiming soldiers had recovered stocks of weapons abandoned by insurgents fleeing the recaptured town of Baga.
Nigerian military claims to have re-taken Baga from Boko Haram came more than a month after it was overrun in what is feared to be the worst massacre in the six-year insurgency.
There was no independent corroboration that Baga had returned to army control- AFP