Nigeria's president-elect Muhammadu Buhari needs a comprehensive, not just military, plan to defeat Boko Haram, experts said on Thursday, after the former army general vowed to crush the Islamists.

"Military force alone is not enough to annihilate the movement," Nigeria researcher Marc-Antoine Perouse de Montclos, from the Chatham House international affairs institute in London, told AFP.

Nine Chadian soldiers were killed and 16 wounded after being ambushed by Boko Haram fighters in northeastern Nigeria, the Chadian army said Thursday.

A military spokesman said fighting took place on Wednesday about 10 kilometers (six miles) from the town of Malam Fatori, which Niger said was retaken by regional forces fighting the Islamist radicals earlier this week.

U.S. President Barack Obama spoke to Nigerian President-elect Muhammadu Buhari and outgoing leader Goodluck Jonathan on Wednesday, praising both for their role in the country's first democratic transfer of power.

Obama spoke to the men separately to "commend them for their leadership during Nigeria's election," the White House said.

Nigeria's president-elect Muhammadu Buhari on Wednesday vowed tough action against Boko Haram and pledged reconciliation and inclusive government, in a speech after his dramatic election win.

The 72-year-old has put tackling the Islamist militants at the heart of his presidential bid and left no room for doubt, as he was presented with a certificate confirming his victory.

U.S. President Barack Obama praised Nigerian President Goodluck Jonathan on Wednesday for conceding defeat in an election that he said showed "Nigeria's commitment to democratic principles."

"President Jonathan has placed his country's interests first by conceding the election and congratulating president-elect (Muhammadu) Buhari on his victory," Obama said in a White House statement. 

Regional forces have retaken a town in northeastern Nigeria where Boko Haram members were said to have fled after suffering losses elsewhere, Niger's military said Wednesday.

The situation in the town of Malam Fatori near the Niger border had been unclear, with the Nigerian army previously claiming it was under its control.

The international community must step up to support countries facing attacks by Boko Haram militants, accused of a litany of atrocities, African nations told the U.N. Human Rights Council Wednesday.

In a draft resolution presented to the U.N.'s top rights body, the group of African countries called on the international community "to provide an active and multifaceted support to Cameroon, Chad, Niger, Nigeria and other states affected by actions of the terrorist group Boko Haram."

Nigeria's new president-elect Muhammadu Buhari on Wednesday hailed polls that will lead to the first democratic change of power in Africa's most populous nation as "historic" hours after he secured a decisive victory.

In official results released early Wednesday, Buhari won Nigeria's presidential election by 2.57 million votes, defeating incumbent Goodluck Jonathan in a triumph greeted by joyous street celebrations.

Egypt and Nigeria accounted for an "alarming rise" in the number of death sentences handed out around the world in 2014, often on the back of security concerns, Amnesty International said Wednesday.

The London-based rights watchdog also criticised Pakistan for lifting a moratorium on the execution of civilians in the wake of the Peshawar school massacre by Taliban militants in December.

Nigeria's former military leader Muhammadu Buhari claimed a historic election victory in Africa's most populous country on Tuesday, sending thousands of jubilant supporters into the streets chanting "change, change."

Buhari's campaign spokesman said incumbent President Goodluck Jonathan had called the retired general to concede defeat in the most closely fought election in Nigeria's history.