Evidence suggests Saudi Arabia's Crown Prince Mohammad bin Salman and other senior Saudi officials are liable for the murder of journalist Jamal Khashoggi, a UN rights investigator said Wednesday.

There was no immediate reaction from Riyadh which was sent the 100-page report in advance - but the kingdom has regularly denied accusations that the prince was involved.

Agnes Callamard, the UN special rapporteur on extrajudicial executions, called for countries to widen sanctions to include the Crown Prince and his personal assets, until and unless he can prove he has no responsibility.

Khashoggi, a critic of the prince and a Washington Post columnist, was last seen at the Saudi consulate in Istanbul on October 2 where he was to receive papers ahead of his wedding. His body was dismembered and removed from the building, the Saudi prosecutor has said, and his remains have not been found.

"It is the conclusion of the special rapporteur that Mr Khashoggi has been the victim of a deliberate, premeditated execution, an extrajudicial killing for which the state of Saudi Arabia is responsible under international human rights law," Callamard said in her report based on a six-month investigation.

Callamard went to Turkey earlier this year with a team of forensic and legal experts and said she received evidence from Turkish authorities. "There is credible evidence, warranting further investigation of high-level Saudi officials' individual liability, including the Crown Prince's," she said. 

"Indeed, this human rights inquiry has shown that there is sufficient credible evidence regarding the responsibility of the Crown Prince demanding further investigation," she added, urging UN Secretary-General to establish an international probe.