REPUBLIKA.CO.ID, LAGOS -- Nigeria's state oil company has offered to talk to an international watchdog that says it failed to hand over billions of dollars in oil revenues despite government promises to tackle mismanagement and corruption.

President Muhammadu Buhari last year fired senior staff at the Nigerian National Petroleum Corporation (NNPC) and approved a revamp of its structure last month.

But in a report published this week, the Natural Resource Governance Institute (NRGI) said NNPC withheld around two thirds of the $6.3 billion of oil proceeds in the second half of 2015.

NRGI said that was an increase of 12 percent from the proportion kept under the administration led by Buhari's predecessor, Goodluck Jonathan, in 2013 and 2014.