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

Muhammadu Buhari’s new administration won a signal of support from the United States, which indicated it was prepared to increase military aid.

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.

With dancing and the release of white doves symbolizing peace, Nigerians hailed the handover of power in an African nation marked by superlatives: the most populous nation, the biggest oil producer, the largest economy.

Nigeria also confronts the most deadly conflict on the continent – the insurgency by Boko Haram that has 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 Islamic extremists to grow into “a terrifying force,” Buhari pledged to take on Nigeria’s myriad problems.

“We are going to tackle them head on,” he declared.

“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.

The military has freed hundreds of captured women and children in recent weeks as it hemmed Boko Haram into its stronghold in the Sambisa Forest, but there has been no word of the schoolgirls whose abduction brought an international outcry.

U.S. Secretary of State John Kerry was the first foreign official to meet with Nigeria’s new leader after the inauguration, accompanied by head of U.S. Africa Command, Gen. David M. Rodriguez. A senior State Department official said Washington was ready to increase military aid and could quickly send more advisers.

The 72-year-old Buhari had earlier pledged to root out human rights violations by the Nigerian military – abuses that had prevented full military cooperation from the U.S. and Britain.

Departing President Goodluck Jonathan last year halted U.S. training of a battalion of Nigerian troops to fight Boko Haram. No reason was given but his officials had expressed anger at U.S. refusals to sell Nigeria weapons, including helicopter gunships.

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