Fears have been raised that the schoolgirls kidnapped by Boko Haram militants earlier this year are being used as suicide bombers.

The speculation has arisen after a number of female suicide bombings in Nigeria's biggest city of Kano, while a ten-year-old girl wearing explosives was also discovered in Katsina state.

The latest attack came on Wednesday when a female suicide bomber blew herself up at a college in Kano, killing six people. According to reports, the bomber was a female teenager.

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Police officers stand guard at the scene of a female suicide bomb attack outside a school in Kano, Nigeria, on Wednesday

It was the fourth suicide bombing carried out by a female in the city in the past week.

There was no immediate claim of responsibility, although militant group Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, has repeatedly bombed Kano as it radiates attacks outwards from its northeast heartlands.

Government spokesman Mike Omeria said security forces arrested three Boko Haram suspects in Katsina state, two of them female, on Tuesday.

One was a 10-year-old girl who had an explosive belt strapped to her by the others, he said.

Using female suicide bombers in the city appears to be a new tactic of Boko Haram, although they have used them on occasion for years in the northeast.

Two female suicide bombers blew themselves up at a trade show and a petrol station in Kano on Monday, killing one other person and injuring at least six others.

Rescue workers collect clothes at the scene of the female suicide bomb attack outside the school in Kano, Nigeria, on Wednesday

On Sunday, a female suicide bomber killed herself but no one else while trying to target police officers.

Concerns have now been raised the militant group is using the schoolgirls kidnapped earlier this year in Chibok in the suicide bombings.

Former education minister Oby Ezekwesili has warned the kidnapped girls may be 'indoctrinated or coerced into being used as suicide bombers', according to the International Business Times.

On Sunday Ms Ezekwesili tweeted: 'This new trend&serial pattern of "FEMALE SUICIDE BOMBERS" surely should PARTICULARLY worry us. It worries me stiff cos of our #ChibokGirls.'

This new trend&serial pattern of "FEMALE SUICIDE BOMBERS" surely should PARTICULARLY worry us. It worries me stiff cos of our #ChibokGirls. — Oby Ezekwesili (@obyezeks) July 27, 2014

Kano again and again. Female suicide bombers again and again - becoming trend. Our #ChibokGirls still in the enemy den. Are we THINKING? — Oby Ezekwesili (@obyezeks) July 28, 2014

The following day she wrote: 'Kano again and again. Female suicide bombers again and again - becoming trend. Our #ChibokGirls still in the enemy den. Are we THINKING?'

Meanwhile, the Human Rights Writers Association of Nigeria has urged the government to investigate the identity of the suicide bombers, the International Business Times reported.



In a statement, it said: 'In the event that these female suicide bombers are identified to have been the same kidnapped girls then the government should immediately deploy all resources and strategies to bring to an end, once and for all, this shameful scenario since the military have repeatedly stated that they are aware of the whereabouts of the kidnapped Chibok girls.'

In a separate incident on Tuesday, two suicide bombers killed 13 people in attacks on two mosques in the town of Potiskum, in Yobe state in the northeast, medical official Bala Afuwa, who received the bodies at a local hospital, told Reuters by telephone on Wednesday.

About 250 schoolgirls were kidnapped in April by members of the militant group Boko Haram

'Two of my uncles were killed,' said resident Mohammed Abubakar, whose family home is next to one of the mosques that were attacked. 'They had just returned from the mosque.'

President Goodluck Jonathan, who has come under heavy criticism for failing to end the five-year-old rebellion, pledged $500 million on Wednesday towards Nigerians living in states that are worst affected by Boko Haram violence.

Earlier this month kidnapped Nigerian schoolgirls who managed to escape from their Islamic extremist captors were reunited with the president.

He heard tales from some of the 57 who escaped after their abduction on April 15 before assuring them of his determination that those still in captivity ‘are brought out alive’.

Around 57 students managed to flee shortly after they were captured, but a committee investigating their disappearance said 219 of the girls are still missing.

Earlier this month kidnapped Nigerian schoolgirls who managed to escape from their Islamic extremist captors met with President Goodluck Jonathan

According to a mediator working with Boko Haram two of the girls have died of snake bites while around 20 have fallen ill.

Boko Haram is demanding a swap for detained fighters in exchange for the girls.

Most of the schoolgirls are still believed to be held in the Sambisa Forest - a wildlife reserve that includes a mixture of thick jungle and open savannah.

The forest borders on sand dunes marking the edge of the Sahara Desert.

Sightings of the girls and their captors have been reported in neighboring Cameroon and Chad.