Nigeria's President Muhammadu Buhari sought to reassure voters that next year's elections would be free and fair on Monday after the opposition and international observers raised concerns over the way a gubernatorial vote was conducted last week.

Buhari's ruling party candidate was narrowly elected governor of the southwestern state of Osun in a vote on Friday which the main opposition party said was marred by voter intimidation and suggested that the 2019 elections in Africa's top oil producing country were unlikely to be fair.

Observers from Britain, the United States and the European Union said in a joint statement that they had witnessed widespread incidents of interference and intimidation of voters, journalists, and civil society observers by some party supporters and security agencies in Osun.

In a televised address to mark the anniversary of the west African country's independence from British rule, Buhari, who will seek a second term in February 2019 elections, tried to address concerns over Nigeria's Independent National Electoral Commission (INEC), which the opposition has said was partly to blame for the manner in which the Osun vote was conducted.

"I have committed myself many times to ensure that elections are fully participatory, free and fair and that the Independent National Electoral Commission will be exactly independent and properly staffed and resourced," said Buhari in his address.

He did not give further details.

Elections in Africa's most populous nation have for years been marred by allegations of irregularities including vote rigging, voter intimidation and violence.

On Saturday Buhari's ruling All Progressives Congress party nominated him as its candidate to stand for re-election next year. The main opposition People's Democratic Party (PDP) has yet to select a candidate.

Friday's Osun poll was a re-run after the INEC declared the first vote, in which the main opposition candidate secured a win, as inconclusive due to the high number of spoiled ballots.

PDP members criticised INEC for its handling of the election and accused it and security agencies of failing to ensure the second vote was free and fair.

INEC's board members are chosen by the president subject to parliament's approval. An INEC spokesperson did not respond to request for comment.