Lagos - Boko Haram is using female suicide bombers to sow wider panic and fear across the country as well as gain greater publicity for its cause, experts said, after two fresh attacks in a week.
The attacks, in Azare in the northeastern state of Bauchi and the Niger state town of Kontagora in the northwest, came after four attacks in a week in the northern city of Kano in July.
In June, another woman was said to have blown herself up in a twin bombing in the southwestern city of Lagos, although her involvement was never confirmed by the authorities.
Earlier that month, Nigeria's first female suicide bombing saw a middle-aged woman detonate her explosives at a military barracks in the northern state of Gombe.
"Normally the use of female suicide bombers is associated with decline in the group, recruitment issues, struggling to find men," said Elizabeth Pearson, from the Nigerian Security Network.
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"Women represent a last resort But this was coming at the time Boko Haram had had its best year probably ever in terms of violence and the numbers killed," she told AFP.
"These attacks came mostly before the announcement of a caliphate (on August 24)," she said, suggesting that the attacks were part of a wider, planned strategy.
The location of the attacks were all outside the three northeast states where Boko Haram is most active.
The group's aim appears to be to raise fears that it has support across Nigeria and the ability to activate sleeper cells at will.
"Boko Haram has got a tendency to propaganda. This is a good way of doing that," Pearson said.'Black widows' and coercion
Suicide bombing is a well-established Al-Qaeda tactic and has been used throughout the Middle East but most attacks are perpetrated by men.
Female suicide bombers, who are rarer but grab more headlines, have been used by Tamil Tiger rebels in Sri Lanka, Turkey's separatist Kurdistan Workers' Party (PKK) and Chechen militants.
Researchers into the phenomenon have said women are more likely to be driven to attack by personal grievances such as the death of family member or husband.
So-called "black widows" made up about a third of all suicide bombers in Chechnya, targeting specific individuals blamed for the death or injury of family members, said anthropologist Scott Atran.
In Sri Lanka, the Tamil Tigers found women easier to infiltrate targets, he added, pointing to the 1991 assassination of former Indian prime minister Rajiv Gandhi by a female suicide bomber.
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Pearson suggested some women, including in deprived northern Nigeria, may be just as ideologically driven as men, although there have been no "martyrdom" videos to explain their motivations.
The age of the Kano bombers and a 10-year-old found in Katsina state in July wearing a suicide vest indicates, however, that some may be forced into bombing.
Forced conscription of young men and boys by Boko Haram has been well-documented in the five years since its insurgency began.
Three women said to be "female recruiters" for Boko Haram were reportedly arrested in July while an alleged trainer of women bombers was detained in Kano in August with up to 16 "trainees".
"It doesn't seem beyond the realms of possibility that they have also been recruiting female suicide bombers at the same time," said Pearson, from King's College, London. 'No different from men'
The young ages of the women used in the Kano attacks prompted speculation at the time that Boko Haram could be using some of the 219 schoolgirls it has held hostage since mid-April.
One Nigerian charity worker even claimed to a US House of Representatives hearing in September that one of the girls had been identified as involved in the July strikes in Kano.
But the Kano police and elders from the girls' hometown of Chibok have denied any links, while Boko Haram's leader Abubakar Shekau, who has gloated about the kidnapping, has not mentioned it.
For Atran, who has profiled suicide bombers, Boko Haram's use of women is significant because of the types of targets.
Boko Haram's first suicide attack was against the UN headquarters in Abuja in 2011, while last week a bomber blew himself up at a school in the northeast town of Potiskum, killing 58.
"So far, all the Boko Haram women have targeted large groups, schools and government buildings exactly as if they had been male," Atran said in an email exchange.
"The Boko Haram women suicide bombers seem no different than the men, and that in itself is noteworthy."