Human Rights Watch on Friday urged Saudi Arabia to allow independent observers access to detained women's rights activists, saying Riyadh's assurances of their well-being could not be trusted following the murder of dissident journalist Jamal Khashoggi.

The kingdom last month denied as "false" and "unfounded" reports published by HRW and Amnesty International that three women activists had been tortured and sexually harassed in detention.

"Saudi Arabia's consistent lies about senior officials' role in Jamal Khashoggi's murder mean that the government's denials that it tortured these women activists are not nearly good enough," said HRW's deputy Middle East director, Michael Page.

Khashoggi, a dissident columnist who lived in self-imposed exile in the United States, was killed inside the kingdom's consulate in Istanbul in early October.

His murder has put mounting pressure on Riyadh and Saudi Crown Prince Mohammed bin Salman, who Turkish officials -- and reportedly the CIA -- have concluded gave the orders.

The New York-based watchdog said it received a new report on November 28 from an "informed source" indicating that Saudi authorities had tortured and sexually harassed a fourth woman activist.

Sources told HRW the torture of Saudi women activists "may be ongoing".

"Unless independent monitors are able to confirm the women activists' well-being, there is every reason to believe that the Saudi authorities have treated them with unspeakable cruelty," Page said.

More than a dozen activists were arrested in May -- just before the historic lifting of a decades-long ban on women drivers the following month.

Interest in the rights campaigners heightened after Canada called for their "immediate release", sparking a diplomatic row that saw Riyadh expel the Canadian ambassador in early August and impose a raft of other sanctions.