DALORI (Nigeria) - Hundreds of women and girls captured by Boko Haram have been raped, many repeatedly, in what officials and relief workers describe as a deliberate strategy to dominate rural residents, and possibly even create a new generation of Islamist militants in Nigeria.

In interviews, the women described being locked in houses by the dozen, at the beck and call of fighters who forced them to have sex, sometimes with the specific goal of impregnating them.

"They married me," said Ms Hamsatu, 25. She said she was four months pregnant, that the father was a Boko Haram member, and that she had been forced to have sex with other militants who took control of her town.

Boko Haram, a radical Islamist sect that has taken over large stretches of territory in Nigeria's north-east, has long targeted women, to be given to Boko Haram fighters for "marriage", a euphemism for the sexual violence.

Now, dozens of newly freed women and girls, many of them pregnant and battered, are showing up at a sprawling camp for the displaced outside the Borno state capital, Maiduguri, as Nigerian soldiers and other military forces try to push Boko Haram out of nearby territory it has occupied for much of the last year.

The full human toll of that occupation is only now emerging. More than 15,000 people have sought shelter at the camp, most of them women, relief officials said. Over 200 have so far been found to be pregnant, but relief officials believe many more are bearing unwanted children.

"The sect leaders make a very conscious effort to impregnate the women," said Borno governor Kashim Shettima. "Some of them even pray before mating... to make the products of what they are doing become children that will inherit their ideology."

The militants have openly vowed to treat women as chattel. After Boko Haram militants kidnapped nearly 300 schoolgirls from the village of Chibok last year, the group's leader called them slaves and threatened to "sell them in the market".

"We would marry them out at the age of nine," the leader, Abubakar Shekau, said in a video message, prompting the global "Bring Back Our Girls" campaign.

As the group lost control of towns, and thousands of people fled in recent weeks, a grim picture has emerged: Hundreds of women and girls as young as 11 had been subjected to systematic, organised sexual violence.

Ms Yahauwa, 30, wiped away tears. She had just tested positive for HIV. Later, she explained that she and many other women had been "locked in one big room".

"When they came, they would select the one they wanted to sleep with," she said. "They said, 'If you do not marry us, we will slaughter you.'"

Six years ago, Nigerian security forces clashed violently with Boko Haram, and the group has waged unremitting war against the federal government ever since.

It recently declared allegiance to the Islamic State in Iraq and Syria, and its successes contributed substantially to the defeat of incumbent president Goodluck Jonathan in March elections.

Boko Haram is now on the retreat, but the countryside is not secure. People from several towns said the militants had not been defeated, as the Nigerian military claims, but simply fled when faced with superior firepower.

Many officials worry about the long-term effects of the abuse.

Ms Yana, a young woman, said the fighters had "parked" her - a word many women used to describe their imprisonment - with about 50 other women in a house in Bama, Borno's second city.

"If they want to have an affair with a woman, they will just take her to a private place, so that the others won't see," she said.

A relief worker at the camp said Ms Yana had been raped so often that she was "psychologically affected". Her feet and stomach were swollen, and the relief worker said she was likely pregnant.

Nigerian officials have reacted gingerly as evidence of large-scale sexual violence by Boko Haram emerges. But officials and relief workers in Borno state, where Boko Haram was born and remains strongest, said the organised nature of the sexual violence appeared to point to a deliberate, self-perpetuation plan.

"It's like they wanted to have their own siblings (sic) to take over from them," said Mr Abba Mohammed Bashir Shuwa, a senior state official in Maiduguri.

NEW YORK TIMES