Lagos - Nigeria's Boko Haram extremists are offering to free more than 200 young women and girls kidnapped from a boarding school in the town of Chibok in exchange for the release of militant leaders held by the government, a human rights activist has told The Associated Press.The activist said Boko Haram's current offer is limited to the girls from the school in northeastern Nigeria whose mass abduction in April 2014 ignited worldwide outrage and a campaign to "Bring Back Our Girls" that stretched to the White House.The new initiative reopens an offer made last year to the government of former President Goodluck Jonathan to release the 219 students in exchange for 16 Boko Haram detainees, the activist said. He spoke on condition of anonymity because he was not authorised to talk to reporters on this sensitive issue.Fred Eno, an apolitical Nigerian who has been negotiating with Boko Haram for more than a year, told the AP that "another window of opportunity opened" in the last few days, though he could not discuss details.He said the recent slew of Boko Haram bloodletting - around 350 people killed in the past nine days - is consistent with past ratcheting up of violence as the militants seek a stronger negotiating position.Presidential adviser Femi Adesina said on Saturday that Nigeria's government "will not be averse" to talks with Boko Haram. "Most wars, however furious or vicious, often end around the negotiation table," he said.Eno said the 5-week-old administration of President Muhammadu Buhari offers "a clean slate" to bring the militants back to negotiations that had become poisoned by the different security agencies and their advice to Jonathan.Two months of talks last year led government representatives and Eno to travel in September to a northeastern town where the prisoner exchange was to take place, only to be stymied by the Department for State Service intelligence agency, the activist said.At the last minute, the agency said it was holding only four of the militants sought by Boko Haram, the activist said.Detainees dyingIt is not known how many Boko Haram suspects are detained by Nigeria's intelligence agency, whose chief Buhari was fired last week.The activist said the agency continues to hold suspects illegally because it does not have enough evidence for a conviction, and any court would free them. Nigerian law requires charges be brought after 48 hours.Thousands of suspects have died in custody, and some detainees wanted by Boko Haram may be among them. Amnesty International alleges that 8 000 detainees have died in military custody - some have been shot, some have died from untreated injuries due to torture, and some have died from starvation and other harsh treatment.In May, about 300 women, girls and children being held captive by Boko Haram were rescued by Nigeria's military, but none were from Chibok. It is believed that the militants view the Chibok girls as a last-resort bargaining chip.In that infamous abduction, 274 mostly Christian girls preparing to write science exams were seized from the school by Islamic militants in the early hours of April 15 2014. Dozens escaped on their own in the first few days, but 219 remain missing.Boko Haram has not shown them since a May 2014 video in which its leader, Abubakar Shekau warned: "You won't see the girls again unless you release our brothers you have captured."In the video, nearly 100 of the girls, who have been identified by their parents, were shown wearing Islamic hijab and reciting the Qur'an. One of them said they had converted to Islam.International indignation at Nigeria's failure to rescue the girls was joined by US first lady Michelle Obama. In a radio address in May 2014, she said she and President Barack Obama are "outraged and heartbroken" over the abduction.There have been unconfirmed reports that some of the girls have been taken to neighbouring countries, and that some have been radicalised and trained as fighters. At least three were reported to have died - one from dysentery, one from malaria and one from a snake bite.Last year, Shekau said the girls were an "old story," and that he had married them off to his fighters.Lawan Zanna, whose daughter is among the captives, said this week that 14 Chibok parents have died since the mass kidnapping, many from stress-related illnesses blamed on the ordeal.Some of the Chibok girls who managed to escape have been rejected by their community and now live with family friends, tired of hearing taunts like "Boko Haram wives."The assumption that all girls and women held by the group have been raped is a difficult stigma to overcome in Nigeria's highly religious and conservative society.Shekau had threatened in 2013 to kidnap women and girls if Nigeria's military did not release detained Boko Haram wives and children. The government freed them in May of that year as a goodwill gesture ahead of failed peace talks.Sex slavesBoko Haram has kidnapped hundreds more - girls, boys, women and young men. Some have become sex slaves, while others are used as fighters, according to former captives.Nigerian opinion on negotiating with the extremists is mixed. Some say the group's crimes are too heinous to be forgiven: The 6-year-old Islamic uprising has killed more than 13,000 people and forced about 1.5 million from their homes."A lot of people take a hard-line stance that you must never negotiate with a terrorist," said Sen. Chris Anyanwu. She called it a "very complex" issue, balancing the lives of more than 200 girls against the dangers of freeing extremists.The militants last year seized a large swath of northeast Nigeria and declared an Islamic caliphate. Nigeria and its neighbours deployed a multinational army that forced them out of towns and villages this year, but the bloodshed has risen at a fierce rate since Buhari's May 29 inauguration amid pledges to crush the insurgency.Eno said that as the president pursues a necessary military solution, he hopes Buhari also understands the need for negotiation.He said the latest opportunity comes through respected Islamic scholars and Muslim elders who were ignored by Jonathan's people but now have taken dangerous and courageous steps to engage the insurgents.