Boko Haram militants are teaching children as young as 13 how to rape their hostages, it has been claimed.

Adult fighters tell their young recruits they will 'have fun' sexually assaulting their captives, according to a boy soldier who fled from the Nigerian jihadists.

The extremists, who have pledged allegiance to ISIS, are reportedly increasingly turning to children to boost their ranks having sustained bad losses in the last year in battles against the Nigerian military.

One boy, abducted by Boko Haram in Baga, Nigeria has described how senior fighters spent two days instructing youngsters how to carry out rapes - repeatedly attacking women and young girls they were holding captive.

Boko Haram militants are teaching children as young as 13 how to rape their hostages, it has been claimed. The jihadist group's leader Abubakar Shekau is pictured, centre

The 15-year-old, named only as Ahmed, revealed how boys were taught to subdue their victims and told not to allow women to 'overpower' them, according to Philip Obaji Jr, writing for the Daily Beast.

Ahmed said: 'The girls will scream and cry for help, but [the militants] didn’t care. Sometimes they’ll be slapped and threatened with guns if they didn't cooperate.'

He added that young fighters were told they would 'have fun' on completion of a successful mission.

A girl who escaped Boko Haram also told the Daily Beast how she had been raped by 'little boys' who were so little she could normally fend them off 'very easily'.

One 'looked like a 13-year-old having sex for the first time' - but succeeded because he had a gun.

The shocking details emerged as it was claimed Boko Haram fighters fleeing an attack on their base last week may have used some of the girls kidnapped in 2014 from northeast Nigeria's Chibok as human shields to prevent being fired upon by fighter jets.

Major General Lucky Irabor, theatre commander of Nigeria's military campaign against the group, showed a news conference aerial footage he said was filmed during the operation in the Sambisa forest that showed Boko Haram fighters moving with women and children.

Boko Haram fighters fleeing an attack on their base last week may have used some of the girls kidnapped in 2014 from northeast Nigeria's Chibok as human shields to prevent being fired upon by fighter jets

"The haggard fighters were just using them as a shield," Irabor told reporters in the northeastern city of Maiduguri. "That is why we did not engage them from the air."

"We had always believed and hoped that going into the Sambisa would afford us the opportunity to get the remaining Chibok girls. What we can't tell is whether those women we can see were the Chibok girls," he said.

Boko Haram militants kidnapped more than 200 girls from their school dormitories in the town of Chibok in April 2014. The first of the girls to be found said most of them were being held in the Sambisa forest, where she was discovered in May.

The group has kidnapped hundreds of men, women and children during its seven-year insurgency aimed at creating an Islamic state in northeast Nigeria, some of whom may also be held in the forest.

The abduction of the Chibok girls, 21 of whom were released in October, brought worldwide notoriety.

President Muhammadu Buhari said on Saturday the Islamist militants' last enclave in the forest, the vast former game reserve in northeast Nigeria that was their stronghold, had been captured.

Reuters has been unable to independently verify that the area has been captured, but the comments from the general were the first reference by a military official to the suspected whereabouts of the girls since Buhari's announcement.

Boko Haram has killed 15,000 people and displaced more than 2 million during its insurgency

The president said the capture of Camp Zero in the forest marked the "final crushing of Boko Haram" but security analysts say the group's ability to carry out attacks in neighbouring Niger, Cameroon and Chad suggests it has multiple bases.

They also say the group split this year with one faction led by Abubakar Shekau operating from the Sambisa forest and the other, allied to Islamic State and led by Abu Musab al-Barnawi, based in the Lake Chad region.

Irabor said the military was pursuing those who fled, adding that 1,240 people suspected of being militants, their relatives or sympathisers had been arrested between December 21 and 28.

Boko Haram has killed 15,000 people and displaced more than 2 million during its insurgency. It controlled an area the size of Belgium in early 2015 but has been pushed back by troops from Nigeria and neighbouring countries since then.