Nigerias 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.

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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.

The NNPC has previously said such accusations failed to account for its costs.

The oil sector in Nigeria, Africa's top crude producer, accounts for about 70 percent of national income.

The constitution requires NNPC to hand over its oil revenue and money is then paid back based on a budget approved by parliament. But the act establishing the company allows it to cover costs before remitting funds to the government.

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On Friday NRGI posted a statement on its website in which it said NNPC had offered to hold talks.

"NNPC has invited NRGI to meetings where NNPC will clarify its position. NRGI appreciates NNPC's open doors and willingness to engage, and will post further updates based on these discussions," it said.

Credit: Reuters