Nigeria's main opposition party on Tuesday expressed concern that swathes of voters could be disenfranchised by Boko Haram violence at next year's elections, undermining the credibility of results.

The country's elections chief conceded last week that a ballot may be impossible in some parts of the northeast worst hit by the violence but it was "unlikely to affect the outcome… nationally".

But Lai Mohammed, spokesman of the opposition All Progressives Congress (APC), said some five million people were registered to vote in Yobe, Borno and Adamawa, worst hit by the violence.

"This is largely an opposition stronghold," Mohammed told AFP. "Five million voters by any standard is huge."

If a large percentage were unable to access polling stations because of unrest or a military lockdown, the APC would "not be comfortable" with the national result, he added.

The head of the Independent National Electoral Commission, Attahiru Jega, said last week that it was "inconceivable" that unrest could prevent voting in throughout all three states.

But even if people in some areas were prevented from voting, it would not affect the credibility of the election, he added.

Nigeria, which is home to 170 million people, on Friday claimed to have brokered a ceasefire with Boko Haram, which wants to create a hardline Islamic state in the country's north.

But violence continued at the weekend and questions have been raised about the credibility of the group's purported negotiator.

The APC is a new coalition of the main opposition parties that ran against the ruling Peoples Democratic Party (PDP) in 2011.

Mohammed said its broad base would make the February 14, 2015, vote "like no other election in Nigeria's history", with APC governors currently controlling 14 of the country's 36 states.

Analysts have warned that primaries to choose a presidential candidate could fracture the party.

Major players such as former military dictator Muhammadu Buhari and former vice president Atiku Abubakar are already in the race.

The APC is expected to select an ethnic Hausa from the mainly Muslim north to run against President Goodluck Jonathan, a southern Christian.

Jonathan faced intense internal pressure to step aside in favour of a northerner to honour an unwritten rule on rotating the country's leadership.

But the president appears to have fought off his rivals and is expected to declare his re-election bid soon.

Mohammed said the APC would consider "the religious sensibilities of Nigerians" when picking its candidate but that tribe and faith were not the key factors.

"We will work towards what we think is best for the country," he said. "It is not a matter of 'it must be north' or 'it must be south'."