ABUJA (REUTERS) - The secretary-general of oil producers' group Opec, Dr Mohammed Barkindo, has died, the boss of Nigerian National Petroleum Corp (NNPC) announced on Wednesday (July 6).

Dr Barkindo, 63, was due to step down at the end of this month after six years in the top job at the Organisation of the Petroleum Exporting Countries (Opec).

"We lost our esteemed Dr Mohammed Sanusi Barkindo," NNPC CEO Mele Kyari wrote on Twitter, adding that he died late on Tuesday.

The death is a "great loss to his immediate family, the NNPC, our country Nigeria, the Opec and the global energy community", Mr Kyari added.

Mr Kyari said Dr Barkindo died hours after meeting Nigerian President Muhammadu Buhari and giving the main speech at an energy summit in Abuja.

Dr Barkindo said the oil and gas industry is "under siege" due to years of under-investment and that allowing trade in oil from Iran and Venezuela could help address the market's current tight supply.

Dr Barkindo's career in the oil industry began in Nigeria in the early 1980s.

He served in various capacities at the NNPC and represented Nigeria on Opec's Economic Commission Board.

He served as acting Opec secretary-general in 2006 before returning to the position 10 years later and has led the organisation through a turbulent oil market period, including steering it towards greater cooperation with non-Opec oil producers.

After leaving Opec, Dr Barkindo was due to join US think tank the Atlantic Council's Global Energy Centre as a distinguished fellow, the council recently announced.