Nigeria's president on Friday sacked the entire board of the corruption-ridden state-run oil company, in what observers said was a prelude to an in-depth probe of the firm's activities.

An official statement from the head of the civil service said Muhammadu Buhari had "dissolved the board of the Nigerian National Petroleum Corporation (NNPC) with immediate effect".

Buhari, who took office on May 29 after a historic win in elections two months earlier, has vowed to tackle what he has called "the evil of corruption".

The oil sector is seen as the worst affected, with claims that billions of dollars have been misappropriated instead of being remitted into government coffers.

"The sacking of the NNPC board was overdue because‎ the fusty smell and stench coming out from that place has been choking all of us in the country," lawyer and rights activist Festus Keyamo told AFP.

Nigeria is Africa's biggest oil producer. Sales of crude account for 90 percent of foreign exchange earnings and about 70 percent of overall revenue.

The NNPC board is statutorily chaired by the petroleum minister.

Buhari, 72, has yet to appoint senior cabinet ministers and it has been suggested that he wants to take charge of the oil and gas portfolio himself to supervise much-needed reforms.