Robert Sirleaf, the son of Liberian President Ellen Johnson Sirleaf who resigned as chairman of the state oil company this week, says his mother never heeded his advice while serving in government.

Speaking to a call-in radio program Wednesday, Robert Sirleaf said his mother had ignored his counsel "from day one." He declined to name the specific issues on which they disagreed.

He said, however, that the main reason he quit was because his work was "completed," echoing a public statement issued by the president on Tuesday.

President Sirleaf, in power since 2006, had three sons serving in top government posts prior to the resignation of Robert Sirleaf, who doubled as a senior presidential adviser. She has repeatedly denied allegations of nepotism, describing her sons as qualified.