Nigeria's outgoing president Goodluck Jonathan on Tuesday fired the country's police chief, a surprise move less than six weeks before leaving office that raised questions of a political motive.

A State House press release carried the headline "President Jonathan sacks Suleiman Abba", and said the police chief had been relieved of his duties "with immediate effect".

No reason was given for the dismissal but the wording was unusually strong compared to similar past statements, which typically thank outgoing security chiefs for their service.

The pro-opposition Nation newspaper on Saturday ran a front-page story claiming Jonathan was facing pressure from his loyalists to fire Abba over his handling of recently concluded elections.

Jonathan lost the March presidential vote to Muhammadu Buhari, the first time an incumbent has been defeated at the polls in Nigeria's history.

Buhari's All Progressives Congress (APC) party also made huge gains in regional elections, claiming a majority of governorship seats in Nigeria's 36 states.

"The timing of (Abba's) sacking, especially a few weeks after a general election in which Jonathan lost, is food for thought," Debo Adeniran of Coalition Against Corrupt Leaders lobby group, told AFP.

"It is even more intriguing because Abba still has some 10 years to go before retirement," he added.

Adeniran suggested Abba may have made enemies by refusing "to carry out the bidding of Jonathan and the (ruling) Peoples Democratic Party to rig the election".

The March vote was praised worldwide for being largely peaceful and credible and there were no significant reports of police misconduct, unlike in past Nigeria elections.

Police spokesman Emmanuel Ojukwu was not immediately available to comment the removal of Abba, who was only appointed in August last year.

Buhari will be sworn in on May 29 and is widely expected to nominate his own choices to fill Nigeria's top security posts.

Jonathan on Tuesday named deputy police chief Solomon Arase as the acting head of the force.