Nigeria's new president was sworn in Friday and pledged to tackle Boko Haram "head on," asserting the fight against the Takfiri extremists wouldn't be won until hundreds of schoolgirls abducted last year and other kidnapping victims were brought home alive.

The inauguration turned into a nationwide celebration by Nigerians welcoming their country's newly reinforced democracy after Buhari became the first candidate to defeat a sitting president at the polls since the end of military rule in 1999.

Nonetheless, Nigerians hailed the handover of power in an African nation marked by superlatives: the most populous nation, the biggest oil producer, the largest economy.

However, Nigeria also confronted the most deadly conflict on the continent - the insurgency by Boko Haram that had killed more than 13,000 people and driven more than 1.5 million from their homes.

Blaming official bungling, negligence, complacency and collusion for allowing the Takfiri extremists to grow into "a terrifying force," Buhari pledged to take on Nigeria's myriad problems.

"But we cannot claim to have defeated Boko Haram without rescuing the Chibok girls and all other innocent persons held hostage by insurgents," he said, referring to the hundreds of girls seized more than a year ago from their school in Chibok in northeastern Borno state.

"This government will do all it can to rescue them alive" he added.

Though, the military had freed hundreds of captured women and children in recent weeks as it hemmed Boko Haram into its stronghold in the Sambisa Forest, there had been no word of the schoolgirls.

Furthermore, the 72-year-old Buhari had earlier pledged to root out human rights violations by the Nigerian military.

Buhari addressed those concerns Friday, promising to overhaul rules of engagement to prevent abuses and to take "disciplinary steps" against violators of human rights.

He also thanked the leaders of neighboring Cameroon, Chad and Niger for sending troops for a multinational offensive that this year had driven Boko Haram from towns where it had declared a Takfiri caliphate.

Suicide bombings, abductions and hit-and-run attacks continue in northeastern Nigeria by what Buhari called "a mindless, godless group as far away from Islam as you can think of."