The son of Liberian President Ellen Johnson Sirleaf has resigned as chairman of the West African nation's state oil company

Robert Sirleaf also stepped down from his role as a senior adviser to his mother.

President Sirleaf has come under pressure from opponents who accuse her of corruption, nepotism and mismanagement of Liberia's resources sectors, allegations she has denied.

In a press conference on Tuesday, President Sirleaf said Robert Sirleaf's resignation had nothing to do with accusations of favoritism. She said he had simply completed his assignment to restructure NOCAL and draft a petroleum law.

"We made a promise that he was there for something specific, and when that assignment was done, we would leave. This was the promise we made. Perhaps people did not believe us. They have criticized. But we have kept that promise," she said.

Fred Bass, vice-chairman of NOCAL's board of directors, will take over as acting chairman.

Elected in 2006, President Sirleaf - a Nobel Peace Prize winner and Africa's first freely elected female president - has won international acclaim for turning around a country devastated by 14 years of sporadic civil war that ended in 2003.

Since then, Liberia's enormous resource wealth has attracted a flood of interest from foreign investors. The government, which is in the process of overhauling its petroleum and mining laws, has signed offshore deals with Chevron Petroleum and Exxon Mobil.

However, corruption is seen as a big obstacle to development in Liberia, which remains one of the world's poorest countries a decade after the war's end.

Almost all of the $8 billion worth of resource contracts signed by Liberia since 2009 have violated its laws, according to a draft audit report commissioned by the government and seen by Reuters earlier this year.

Despite declaring a zero tolerance policy for corruption, President Sirleaf has been criticized for appointing three of her sons to government posts.

Her son Fumba Sirleaf is head of the National Security Agency.

In October, the president suspended her son Charles from his position as deputy governor of the central bank and 45 other government officials for failing to declare their assets to anti-corruption authorities.