The Secretary General of the OPEC oil cartel, Nigeria's Mohammad Barkindo, has died unexpectedly at the age of 63, the Nigerian National Petroleum Corporation said today.

"We lost our esteemed Dr (Mohammad) Sanusi Barkindo. He died at about 11pm yesterday 5th July 2022," NNPC's chief executive Mele Kyari tweeted.

He did not provide any further details about the circumstances of Barkindo's death.

"Certainly a great loss to his immediate family, the NNPC, our country Nigeria, the OPEC and the global energy community. Burial arrangements will be announced shortly," Kyari wrote.

"This tragedy is a shock to the OPEC Family," the Vienna-based Organization of the Petroleum Exporting Countries tweeted.

"We express our sorrow and deep gratitude for the over 40 years of selfless service that Barkindo gave to OPEC. His dedication and leadership will inspire OPEC for many years to come," OPEC added.

Only a few hours earlier, Barkindo had met Nigerian president Muhammadu Buhari in Abuja, where Buhari praised Barkindo's "distinguished" achievements at OPEC.

"You were able to successfully navigate the organisation through turbulent challenges," the president said.

Barkindo had headed OPEC since 2016 and was scheduled to be replaced by Kuwait's Haitham Al-Ghais next month.

The cartel's secretary general does not wield any executive power, but is OPEC's public face and frequently has to act as a diplomat to bring together the different, sometimes conflicting interests of the various member countries.

Under Barkindo's stewardship, OPEC forged ties with 10 other oil producing countries, such as Russia, to form a wider group known as OPEC+ in a bid to better tame global oil prices, rocked by the challenges of the coronavirus pandemic and more recently the war in Ukraine.