Three suicide bombings by girls aged as young as 10 suggest that Nigeria's Boko Haram has employed a new tactic of forcing abducted children to blow themselves up, according to experts.

The Islamist sect has been carrying out almost daily killings and kidnappings across northeast Nigeria in a campaign of violence now in its sixth year. Deadly attacks on Saturday and Sunday were carried out by three young female suicide bombers.

These came just days after a week-long killing spree by Boko Haram, in which the group torched at least 10 towns leaving around 2,000 people unaccounted for. Secretary of State John Kerry on Thursday called the attack "a crime against humanity."

It is not clear if the girls were coerced or were even aware they were strapped with explosives, which may have been detonated remotely. But experts say that Boko Haram appears to be using the children it kidnaps — such as the 276 Chibok girls who sparked the #BringBackOurGirls social media campaign — and using them as a readily available supply of suicide bombers.

"Using children to carry and detonate explosives is not a new tactic for Boko Haram but it is an intensification. Boko Haram has been abducting and conscripting children and young men and women for a long period for various purposes – they will be seen by the movement as expendable resources," said Elizabeth Donnelly, assistant head of the Africa program at London's Chatham House think tank.

Boko Haram roughly translates to "Western education is sinful." The group aims to create its own state based on strict Islamic law.

It has used female suicide bombers before, notably in a spate of attacks last July in central northern city of Kano. But Peter Pham, director of the Africa Center at the Atlantic Council, said the recent use of young girls represented a "ratcheting up" of the group's bombing campaign, "first to women and now to children."

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"Young women who are abducted, it has been suggested that they might be used as wives. But it's certainly possible they could be used as bombers," he added.

"These girls just do not matter to them ... They are disposable bodies"

Increasing violence in Nigeria's northeast means reporting from the region has become next to impossible. Western journalists rarely get closer than the capital Abuja, 400 miles away.

But a reporter in the city of Maiduguri, the site of Saturday's bombing, characterized the use of girls by Boko Haram as "clearly a change in tactics."

"Getting the girls? Extremely simple," the journalist told NBC News, asking not to be named because of restrictions imposed by his employer. "Many towns and villages have been raided, and married women, young girls and even children below 10 have been abducted. These are the suicide bombers."

Eyewitnesses told Reuters news agency and local newspapers that the bomb in Maiduguri was strapped to a girl who appeared as young as 10 years old. Local Senator Ahmed Zanna told NBC News around 20 people were killed. The following day, two girls described as appearing the same age wore suicide vests that detonated in the town of Potiskum killing three people.

United Nations Secretary-General Ban Ki-moon condemned what he called a "depraved act."

A man injured in a suicide blast is transported by ambulance at the General Hospital in Potiskum, Nigeria, on Sunday. AMINU ABUBAKAR / AFP - Getty Images

According to Barrister Zannah Mustapha, a local philanthropist who set up a school for orphans in Maiduguri, impoverished children in the area are particularly vulnerable to Boko Haram.

"For a young girl to use a suicide bomb, some sort of indoctrination has gone on," he told NBC News via telephone. Maidurguri has been inundated by people fleeing Boko Haram from other parts of the state. Mustapha said there were currently 2,000 orphans who had applied to attend his school, The Future Prowess Islamic Foundation, which has a current capacity of 420 children.

"These are potential targets for Boko Haram to indoctrinate," he said. "If we cannot accommodate them, where can they go?"

Boko Haram was involved in 10,000 violent deaths last year, according to figures from the Washington, D.C.-based Council on Foreign Relations think tank.

On Thursday, Kerry spoke about last week's incident in which Boko Haram is feared to have killed as many as 2,000 people in and around the fishing town of Baga. "It’s an enormously horrendous slaughter of innocent people," he said. "Boko Haram continue to present a serious threat not just to Nigeria and the region but to all of our values."

The number of young girls kidnapped by Boko Haram is near-impossible to estimate. The group's most high-profile kidnapping came in April last year, which it took 276 girls from a school in Chibok.

Donnelly said abducted girls have provided the group with a resource of bombers it sees as completely expendable.

"I think it's because these girls just do not matter to them," she said. "They are disposable bodies ... How do you fight this? It is hard to identify someone as a Boko Haram fighter when they are using children."

Reuters contributed to this report.