Amnesty International says at least 2,000 women and girls have been kidnapped by Takfiri Boko Haram militants in Nigeria since the beginning of 2014, and many of them have been forced into sexual slavery or combat.

In a report released on Tuesday, the rights group said it had documented 38 cases of abduction by the militants, based on accounts by witnesses and the women and girls who managed to escape their captors.

“It is difficult to estimate how many people have been abducted by Boko Haram,” Amnesty said in the report, titled “‘Our job is to shoot, slaughter and kill:’ Boko Haram’s reign of terror,” which was released on the first anniversary of the abduction of more than 270 Nigerian school girls.

In April 2014, Boko Haram kidnapped some 276 girls from their dormitories at the Government Girls Secondary School in the remote town of Chibok in the volatile Borno State in northeastern Nigeria.

Fifty seven of the girls managed to escape, but 219 are still missing, and international efforts to locate and rescue them have failed so far.

“The evidence presented in this shocking report, one year after the horrific abduction of the Chibok girls, underlines the scale and depravity of Boko Haram’s methods,” said Salil Shetty, Amnesty International’s secretary general.

The Amnesty report quoted a senior military source as saying that the school girls had been split into three or four groups, and were being held at different Boko Haram camps, including in the Sambisa Forest stronghold in Borno, around Lake Chad, and the Gorsi mountains in Cameroon, while about 70 girls were also thought to be in Chad.

Meanwhile, on Tuesday, Nigeria's President-elect Muhammadu Buhari pledged to make every effort to secure the release of the schoolgirls, but admitted it was not clear whether they would ever be found.

“We do not know if the Chibok girls can be rescued. Their whereabouts remain unknown. As much as I wish to, I cannot promise that we can find them,” said Buhari, who won the presidential election two weeks ago.

The kidnapping of women and girls are among crimes - including crimes against humanity and war crimes - documented by Amnesty researchers, who interviewed nearly 200 people, including 28 women and girls who escaped captivity.

Amnesty's report sheds new light on the brutal methods used by Boko Haram, including mass kidnappings, cataloging the frequent abduction of young women and girls, gang rapes as well as the forced recruitment of men and boys.

“Men and women, boys and girls, Christians and Muslims, have been killed, abducted and brutalized by Boko Haram during a reign of terror which has affected millions,” said Shetty.

According to Amnesty, at least 5,500 civilians, were killed by Boko Haram as it rampaged across northeast Nigeria during 2014 and early 2015.

Boko Haram, whose name means “Western education is forbidden” says its goal is to overthrow the Nigerian government.

The Takfiri militants have claimed responsibility for a number of deadly shooting attacks and bombings in various parts of the country since the beginning of their militancy in 2009.