Nigeria's military on Tuesday stepped into a political row about the legitimacy of the main opposition candidate's run for the presidency, in a fresh twist to an issue gripping the campaign.

The ruling Peoples Democratic Party (PDP) has raised doubts about whether President Goodluck Jonathan's main challenger, Muhammadu Buhari, obtained his school-leaving qualification.

Under Nigerian law presidential candidates must have the certificate as a minimum requirement to run for office.

Next month's vote is expected to be closely fought, and Buhari's All Progressives Congress (APC) has been seen as having its best chance of seizing power for the first time since civilian rule was restored in 1999.

The former army general, who headed a military government for 20 months from December 1983 after seizing power in a coup, has dismissed the claims as "desperation".

But the Nigerian army has now denied the 72-year-old's assertion that it has copies of his academic credentials.

"It is a practice in the Nigerian Army that before candidates are shortlisted for commissioning into the officers' cadre of the Service, the Selection Board verifies the original copies of credentials that are presented," said spokesman Olajide Laleye.

"However, there is no available record to show that this process was followed in the 1960s."

He added: "Neither the original copy (of his senior school certificate), certified true copy, nor statement of result... is in his personnel file."

The only records available are his application to join the military on October 18, 1961, and a report from his school principal recommending him as suitable for service, he added.

The APC immediately accused the PDP and the government of "playing a dangerous game by trying to compromise the military in order to satisfy selfish political objectives".

"We know that the PDP and the Jonathan administration will like nothing more than the disqualification of our candidate so they will face no challenge in next month's election," said party spokesman Lai Mohammed.

He added: "What really is happening in our country if some politicians have now decided to engage in an action that will amount to dragging the military into politics?"

Buhari, who is expected to address a news conference on the issue early Wednesday, has come under increasingly personal attacks from the PDP about his state of health and religious views.

Some analysts have seen that as a smear campaign designed to derail his campaign and deflect attention from the government's record in power in the last four years.

Buhari has said he was allowed to run for president on three previous occasions (in 2003, 2007 and 2011) and the electoral body found nothing untoward.

"Really this desperation of disinformation that is being passed around will do nobody any good because our minds are being taken away from the serial issues of corruption and incompetence by the PDP," he has said.