President Teodoro Obiang Nguema, who has ruled the oil-rich country with an iron fist for 39 years, fired Colonel Ruslan Hermes Nguema Oyana for "irregularities committed in the exercise of his functions," said a presidential decree issued Thursday evening.

Obiang, 76, seized power in the small former Spanish colony in 1979 and has faced a string of coup attempts during nearly four decades in office.

In January he fired four senior regime officials, including his grandson, on suspicion of collusion with a foiled December 2017 "coup".

Critics accuse him of brutal repression of opponents as well as election fraud and corruption.

Oyana's deputy, Division General Lamberto Nguema Micha, has replaced him.

He had only been appointed to the post on October 12, on the 50th anniversary of the country's independence from Spain. Several other officers promoted on the same day have now also been demoted, military sources told AFP.

Among those promoted in October was the son of the president, Teodorin Nguema Obiang, vice president with responsibility for defence and security, and several other members of the presidential family, said the same sources.

He was not among those demoted along with Oyana, said the military sources.

Teodorin Nguema Obiang was promoted from colonel directly to division general, without passing through the normal intermediary rank of brigade general.

A French court in October 2017 convicted the president's son of looting public funds from his country to fund his playboy lifestyle in Paris and handed down a suspended fine of 30 million euros and a suspended three-year jail term.