UN human rights experts have urged Saudi authorities to release activist Jane Hathloul, who has been in detention for 500 days and her imprisonment marked the start of a "fierce" campaign by Saudi authorities against women human rights defenders.

The experts said that the charges against Hathloul seeking to undermine state security are malicious, highlighting Hathloul's efforts that have contributed to empowering women with the right to drive cars.

Experts described the continued detention of Hathloul as a "scandal" because it called for changing some laws that the Saudi authorities themselves resorted to change.

The experts added that the human rights gains achieved for women in Saudi Arabia should not prevent the international community from following up the existing human rights problems in the Kingdom.

The experts made an urgent appeal for the release of al-Hathloul and all other prisoners of conscience in Saudi Arabia, and an investigation was opened to determine whether al-Hathloul was tortured during her detention.

In August, the family of Saudi activist Jane Hathloul said her daughter had rejected an offer to release her in exchange for a video statement denying reports that she had been tortured in detention.

Walid al-Hathloul, brother of Jane, 30, said on his Twitter page that she had initially agreed to sign a document denying that she had been tortured and harassed. Her family has been silent recently in the hope of resolving the issue in secret.

But Walid added that state security asked her in a recent interview to record the denials in a video as part of her release agreement. Walid said in a tweet that appearing in a video that Lujain had not been tortured was an unrealistic demand.

Some activists have appeared in court this year on charges of working in defense of human rights and contacting foreign journalists and diplomats, but the hearings have not been held for months.

The case has drawn international criticism and anger in European capitals and the US Congress, especially after the killing of journalist Jamal Khashoggi by Saudis inside the Saudi consulate in Istanbul on October 2 last year.