White House officials say US President Barack Obama has encouraged Nigerian leaders to work together to unify the country and build on recent progress against Boko Haram.
Hundreds of Boko Haram militants in northern Nigeria have been killed by armed forces from neighbouring Chad, an army spokesman has said.
Col Azem Bermandoa said Chadian forces were carrying out mopping-up operations in northern areas around the border town of Malam Fatori, which was recently claimed back from by the Islamist extremist group.
Nine Chad soldiers were killed and 16 injured in the fighting, he added.
The newly-elected President of Nigeria, Muhammadu Buhari, has vowed his government will "spare no effort" in the battle against Islamist militants Boko Haram.
In his first formal speech since his victory against opponent Goodluck Jonathan, Buhari said:
Boko Haram will soon know the strength of our collective will. We should spare no effort.
In tackling the insurgency, we have a tough and urgent job to do.
Nigerian election winner Muhammadu Buhari says his country proved it has "embraced democracy" after his historic victory.
Buhari, 72, became the first politician to unseat a sitting leader at the ballot box yesterday.
"We have proven to the world that we are people who have embraced democracy - we have put one-party state behind us," he said.
Mr Buhari also paid tribute to outgoing president Goodluck Jonathan for relinquishing power, telling journalists: "President Jonathan was a worthy opponent and I extend the hand of fellowship to him."
Nigerian President Goodluck Jonathan has conceded defeat in the country's election, urging supporters to vent their frustrations peacefully following the loss.
Former military ruler Muhammadu Buhari, 72, became the first opposition candidate to oust a president in a democratic election, following a tightly-fought contest.
"Nobody's ambition is worth the blood of any Nigerian," he said as he congratulated his successor.
"The unity, stability and progress of our dear country is more important than anything else."
The new First Lady of Nigeria has hailed a "triumphant show of democracy" after her husband and opposition leader Muhammadu Buhari won the country's election.
All Progressives Congress leader Buhari, who said he had been "converted" to democracy after first seizing power three decades ago in a military coup, is the first Nigerian to oust a sitting president through the ballot box.
His wife, Aisha Buhari, took to Twitter to share her joy as the results rolled in, saying she was "so proud" of her husband and his team.
I am excited for the future of this nation as We now have a mandate for Change.
We must see this as a triumphant show of democracy, a change for the better.
Nigerians are aware now more than ever that the people have the power to sway the fate of a nation.
Voters in Nigeria have taken to the streets waving banners and flags in celebration after the opposition All Progressives Congress took power after the country's general election.
APC leader Muhammadu Buhari claimed victory over President Goodluck Jonathan's People's Democratic Party in the election - the first time in the country's history that a sitting government has been ousted in a purely democratic vote.
Celebrations spilled out into the streets as the election results poured in, with young men performing motorbike stunts and dancing as reports emerged claiming Jonathan had conceded defeat, allaying fears he might refuse to relinquish power.
Campaign groups have joined calls from broadcaster Al Jazeera to free two journalists detained at a hotel in Maiduguri after filming for a report on Islamist extremist group Boko Haram.
Ahmed Idris and producer Ali Mustafa - both Nigerian nationals - were working on a series of pieces ahead of the country's general election when they were detained in their hotel room on March 24.
Bosses at Al Jazeera say the coverage was arranged with the full cooperation of the Nigerian military, and say both men were fully accredited to film anywhere in the country during the election period.
But the military has accused them of "operating without protection, accreditation or due clearance", and of "loitering" in various locations.
The detention has been condemned by the Committee to Protect Journalists and the Nigeria Civil Society Situation Room.
Former Nigerian President Goodluck Jonathan has called opposition leader Muhammadu Buhari to congratulate him on his victory in the country's general election, according to a spokesman for the winning All Progressive's Congress party.
At about 5 minutes to 5, President Jonathan called General Muhammadu Buhari, the winner of the elections, to congratulate him.
I think he conceded defeat. There had always been this fear that he might not want to concede but he will remain a hero for this move.
The tension will go down dramatically.
Nigeria's opposition All Progressive's Congress has declared an "historic" election victory for the African nation, becoming the first party to overthrow the sitting government through a democratic vote.
With 35 of 36 states now declared, the APC had 15.1 million votes against 11.7m for the People's Democratic Party, whose leader and former president, Goodluck Jonathan, is now set be replaced by APC's former military leader Muhammadu Buhari.
APC spokesman Lai Mohammed said there was "no reason" to suspect that Jonathan would refuse to concede, after announcing before the election that he would relinquish power if he was voted out in a "free and fair election".
The run-up to the election period has been plagued with violence from Islamist extremist group Boko Haram, which saw a surge in membership during Jonathan's five years in charge.
But bar some technical glitches and a Boko Haram attack which left more than a dozen voters dead, the election has been been the smoothest in most orderly in recent history.