Angolan billionaire Isabel dos Santos, who is facing a massive corruption investigation, stepped down from the board of the country's largest mobile phone operator Unitel on Tuesday, citing a "climate of permanent conflict" among administrators.

Described by Forbes magazine as the wealthiest woman in Africa, she is accused of diverting billions of dollars from state companies during her father Jose Eduardo dos Santos's nearly 40-year rule of the oil-rich southern African nation.

She owns a 25 percent stake in Unitel, which dominates Angola's telecoms industry with 80 percent of the market share.

"After 20 years dedicated to the creation, development and success of Unitel, I chose to leave the position of member of the company's board of directors," she said in statement on Tuesday.

Dos Santos, 47, said she was stepping down as it was "counterproductive and irresponsible to allow a climate of permanent conflict and systematic politicisation of the administrators".

Her assets were frozen by Angolan investigators in December last year as part of a corruption crackdown.

Angolan businesswoman Isabel dos Santos has had her assets frozen as part of vast corruption investigation Angolan businesswoman Isabel dos Santos has had her assets frozen as part of vast corruption investigation Photo: AFP / MIGUEL RIOPA

That set off a flurry of developments, including thousands of "Luanda Leaks" papers alleging she funnelled Angolan state funds to offshore assets.

Angola's former colonial ruler Portugal then froze her assets in that country, where she had invested mainly in the banking and telecommunications sectors.

In January she was accused of a long list of crimes in Angola, including mismanagement, embezzlement of funds, and money laundering during her stewardship of the state-run oil giant Sonangol.

She has vehemently denied the allegations and denounced Luanda's actions as a politically-motivated "witch-hunt".

After losing an appeal to unfreeze her assets, in June she said he had been "denied justice".

She was ousted as Sonangol chair just months after her father stepped down in 2017 and his hand-picked successor, Joao Lourenco, launched the corruption crackdown.