A top aide to Saudi Arabia's crown prince, fired for his role in the killing of journalist Jamal Khashoggi, personally oversaw the torture of at least one detained female activist earlier this year, two sources with knowledge of the matter said.

Saud al-Qahtani was a royal adviser to Crown Prince Mohammed bin Salman until October, when he was sacked and then sanctioned by the US Treasury over the Washington Post columnist's murder.

Now three sources, briefed on the activists' treatment, say a group of men subjected this woman and at least three others to sexual harassment, electrocution and flogging between May and August at an unofficial holding facility in Jeddah.

They described the group of about six men as distinct from the regular interrogators the women saw and said they belonged to the Saudi Federation for Cybersecurity, Programming and Drones, which Mr Qahtani headed at the time, or to state security.

Mr Qahtani was present when at least one of the women was tortured, two of the sources said.

Reuters has been unable to reach Mr Qahtani since he was sacked in October.

Portraits of Saudi King Salman (R) and his son Crown Prince Mohammed bin Salman (MBS) are displayed in Riyadh Credit: FAYEZ NURELDINE/AFP/Getty Images

A Saudi official said the allegations of mistreatment and torture of the female detainees were "false ... and have no connection to the truth."

"The detainees were detained based on accusations related to harming the security and stability of the kingdom," the official said in response to questions from Reuters.

Their legal rights were being respected and they were receiving medical and social care, family visits and had the right to an attorney, the official said.

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The women are among more than a dozen prominent activists arrested since May amid a broader crackdown targeting clerics and intellectuals. Eleven women are still being held, activists say, including the four alleged to have been tortured.

The allegations come as Riyadh tries to get past the Oct. 2 killing of Khashoggi, a long-time royal insider who became a critic of Prince Mohammed and went into self-exile in the United States last year.

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Khashoggi was killed inside the kingdom's Istanbul consulate, damaging the crown prince's reputation and opening Riyadh up to the threat of sanctions.

Amnesty International and Human Rights Watch said last month at least three of the activists -- most of whom had agitated for the right to drive and an end to a male guardianship system -- were tortured. They did not report Mr Qahtani's involvement.

The sources, who asked not to be identified for fear of reprisals, told Reuters that Mr Qahtani was in the room on several occasions when one of the four detained activists was subjected to kissing, groping and electrocution. He made threats of rape and murder to the woman, the sources said.

At least two other detainees endured sexual insults, flogging and electric shocks that turned one of the women's fingers blue, the sources said. Captors also made another woman kiss a male detainee while they watched, one of the sources said.

Reuters could not determine whether Mr Qahtani was in the room during the episodes with those three other detainees, but the sources said all of the women's tormentors were from "Saud's group".

A third source said Mr Qahtani addressed several of these women in May when they were initially transferred to Jeddah from Riyadh, telling them the penalty for treason was 20 years in prison or the death penalty.