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.