Nigeria's former president Olusegun Obasanjo has met with people close to Boko Haram in an attempt to broker the release of more than 200 kidnapped schoolgirls, a source close to the talks told AFP on Tuesday.
Reports of the talks emerged as Boko Haram was blamed for fresh attacks targeting the security forces, public buildings and a school in its northeastern stronghold.
Cameroon also said it had begun deploying 3,000 extra troops to buttress its border with Nigeria against the threat posed by marauding militants.
On Monday evening, Nigeria's Chief of Defence Staff, Air Chief Marshal Alex Badeh, said the 223 girls still missing had been located but cast doubt on the prospect of any rescue by force.
The talks last weekend at Obasanjo's farm in southern Ogun state included relatives of senior Islamist fighters, intermediaries and the former president, the source said on condition of anonymity.
"The meeting was focused on how to free the girls through negotiation," said the source, referring to the kidnapped schoolgirls, whose abduction has triggered global outrage.
Nigeria's response to the mass abduction has been widely criticised and the hostage crisis has brought unprecedented international attention to Boko Haram's five-year extremist uprising.
- Previous attempts at talks -
Obasanjo, who left office in 2007, has previously sought to negotiate with the insurgents, including in September 2011 after Boko Haram bombed the United Nations headquarters in Abuja.
Then, he flew to the Islamists' base in the Borno state capital, Maiduguri, to meet relatives of former Boko Haram leader Mohammed Yusuf, who was killed in police custody in 2009.
The 2011 talks did not help stem the violence and some at the time doubted if Obasanjo was dealing with people with the authority to negotiate a ceasefire.
The former head of state, who remains an influential figure in Nigerian politics, refused to take questions when reached by phone earlier Tuesday.
But the source told AFP that Obasanjo had voiced concern about Nigeria's acceptance of foreign military personnel to help rescue the girls.
"He said he is worried that Nigeria's prestige in Africa as a major continental power had been diminished" by President Goodluck Jonathan's decision to bring in Western military help, including from the United States.
Mustapha Zanna, the lawyer who helped organise Obasanjo's 2011 talks with Boko Haram, said he was at the former president's home on Saturday.
But he declined to discuss whether the Chibok abductions were on the agenda.
"I was there," he told AFP, explaining that Obasanjo was interested in helping vulnerable children in Nigeria's embattled northeast.
Zanna had represented Yusuf's family in a wrongful death lawsuit filed against the government following his death in police custody.
- 'Bungled contact' -
It was not clear if Obasanjo's weekend meeting had been sanctioned by the government.
Obasanjo backed Jonathan's 2011 presidential campaign but fiercely criticised him and his record as president in an open letter last December, and the two are widely thought to have fallen out.
According to the source, Obasanjo supported a prisoner-for-hostage swap that would see some of the girls released in exchange for a group of Boko Haram fighters held in Nigerian custody.
As a private citizen with damaged ties to the presidency, Obasanjo likely does not have the authority to negotiate any deal on the government's behalf.
Abuja has officially ruled out a prisoner swap but sent intermediaries to meet Boko Haram in the northeast to negotiate for the girls' release.
The source identified one of the envoys as Ahmad Salkida, a journalist with ties to Boko Haram who was close to Yusuf before his death.
Salkida did not respond to several phone calls and text messages seeking comment.
"There was contact but it was bungled by the government," according to the source, saying Jonathan backed away from the deal after returning from a security conference in Paris earlier this month.
- Troop movements, fresh attacks -
The conference saw Nigeria and its neighbours vow greater cooperation to tackle Boko Haram because of the potential threat to regional stability.
Cameroon, criticized in Nigeria for providing safe haven to the militants, said that 3,000 troops had been sent to the country's northern border with Nigeria.
"It's a significant number since the number of troops and police currently posted in the region is less than 1,000," a police source told AFP.
Nigeria's chief of defence staff said that despite having located the girls, the risks of storming the area with troops in a rescue mission were too great and could prove fatal for the hostages.
Suspected Boko Haram gunmen raided the town of Buni Yadi after sundown on Monday, attacking the same area where scores of students were massacred in February.
They shot at soldiers manning a checkpoint, razed a police station and torched the home of a local leader and public buildings before opening fire on a primary school.
"When they started attacking, people began to flee. There were casualties on the part of the security personnel but I don't know how many," said witness Kura Babagana.