Nigeria's state-run oil firm on Monday hit back at claims of corruption from a senior government minister that have put President Muhammadu Buhari's war on graft in the spotlight.

The Nigerian National Petroleum Corporation (NNPC) said the allegations made by junior oil minister Emmanuel Ibe Kachikwu were "baseless and due process has been followed".

Kachikwu, a former ExxonMobil executive to whom Buhari handed the day-to-day running of the oil and gas sector, made the claims in a letter to the president that was leaked last week.

In it, he said five NNPC contacts with a total value of $25 billion "were never reviewed or discussed with the board" -- and he had "many more" examples of questionable practices.

NNPC group general manager Ndu Ughamadu said "a review or discussion with the minister of state (Kachikwu) or the NNPC board on contractual matters" was not required.

The row has once again mired the NNPC in controversy, just as the government is trying to overhaul its reputation as a company with murky finances.

But Kachikwu's letter -- dated August 30 -- runs the risk of threatening Buhari's anti-corruption campaign, as there have so far been no official inquiry or sanctions.

The main opposition Peoples Democratic Party (PDP) said last week that a proper investigation from Buhari would be a "sign of impartiality in the war against corruption".

"He should do this to correct the open impression Nigerians have about his so-called anti-corruption war that it's just a tool of persecution of perceived enemies," it added.

Buhari has been repeatedly accused of mounting a political witch-hunt as most of those arrested so far on corruption charges are from the PDP.

The president appointed Maikanti Kachalla Baru as NNPC group managing director last year.

Oil industry figures have seen Kachikwu's letter as a clear broadside against Baru, an NNPC insider who like Buhari is from northern Nigeria.

Buhari has also been accused of running a government along regional lines.

In his letter, Kachikwu -- a southerner -- complained he had not been able to meet Buhari to discuss the matter.

Source: AFP