Nigeria's leader has vowed to do "everything possible" to bring home nearly 300 abducted schoolgirls.

President Goodluck Jonathan also said that the Islamic extremists responsible threaten the country's democratic gains.

"It is a sad fact that as I address you today, all the gains of the past 15 years of democratic governance in our country are threatened by the presence of international terrorism on our shores," he said in a speech marking Nigeria's transformation from decades of military dictatorship.

He also blamed the Islamic uprising on "extremist foreign elements" and offered amnesty to those who renounce violence.

The Nigerian leader gave no details of what is being done to rescue the girls who the military claimed this week it has located. But the military chief said he fears using force to rescue them could instead get them killed.

The president has ruled out swapping the girls, who were kidnapped from a school in the town of Chibok on April 15, for detained insurgents.

Boko Haram started off as a moderate religious sect nicknamed after the shouted refrain of its leader - "Western education is sinful" - who preached that Western influences have corrupted Nigerian society and caused the massive graft that impoverishes the nation and keeps people in the north east among the poorest of the poor.

Jonathan has promised to address the poverty that helps fuel the uprising - but only once the insurgency is put down. The World Bank says two-thirds of 170 million Nigerians struggle in poverty in Africa's biggest oil producer.

Government troops sought to crush the Boko Haram movement in 2009, blasting its compound and mosque in Maiduguri, capital of northeast Borno state. Some 700 people were killed. The captured leader, Mohammed Yusuf, was shot dead in police custody.

The group returned a year later, initially attacking police stations, and today has rear bases across borders in Chad, Niger and Cameroon. The military says fighters from those countries have been found fighting alongside Boko Haram in Nigeria. The insurgents are well-equipped with armored personnel carriers, machine guns and many other weapons, most apparently looted in attacks on Nigerian barracks.

The president offered an amnesty, saying: "For our citizens who have joined hands with al Qaida and international terrorists in the misguided belief that violence can possibly solve their problems, our doors remain open to them for dialogue and reconciliation, if they renounce terrorism and embrace peace."

What Nigeria is experiencing is "a manifestation of the same warped and ferocious world view" that brought down the Twin Towers in New York and killed innocents at the Boston Marathon last year, he said.

Thousands of people have been killed in the five-year-old insurgency, more than 2,000 so far this year, and an estimated 750,000 Nigerians have been driven from their homes.

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