An official Saudi Arabian investigation into the murder of journalist Jamal Khashoggi by Saudi agents failed to examine who may have ordered the killing and ignored key suspects, a UN special rapporteur has said.

Saudi Crown Prince Mohammed bin Salman, a key adviser, and other senior officials should be investigated over the premeditated murder at its consulate in Istanbul given the evidence against them, said Agnes Callamard, UN investigator on extrajudicial executions who undertook her own inquiry.

Saudi officials have long denied suspicions in the CIA and some Western countries that the crown prince, the kingdom's de facto ruler, ordered the killing in October.

"The inquiry, which was a human rights inquiry, not a criminal investigation, has found credible evidence, warranting further investigation, of high-level Saudi officials' individual liability, including that of the Crown Prince of Saudi Arabia and his key adviser (Saud) Qahtani," Ms Callamard told the UN Human Rights Council.

"The investigation carried out by the Saudi authorities has failed to address the chain of command," she said.

She reiterated the recommendation in her report, issued this month, for an international criminal investigation.

Agnes Callamard is a UN investigator on extrajudicial executions

Abdulaziz Alwasil, Saudi ambassador to the UN in Geneva, said Ms Callamard's report was "based on prejudice and prefabricated ideas".

"This is why we reject any attempt to remove this from our national justice system in Saudi Arabia, irrespective of the form that may take," Mr Alwasil told the council.

Gulf allies, including Bahrain and the United Arab Emirates, spoke in favour of allowing the Saudi justice system to do its work.

Mr Qahtani, seen as the right-hand man to Prince Mohammed, was removed as a royal court adviser and is the highest-profile figure implicated in the incident. However, he is not among 11 on trial.

"The prosecutor in a public statement has recognised that one particular person, Saud al-Qahtani, incited the mission before it departed, calling Mr Khashoggi 'a national threat', and yet this particular individual has not been indicted," Ms Callamard said, noting that a total of 15 Saudis were involved.

Walter Stevens, European Union ambassador, said that those responsible must be held to account and called on Saudi Arabia to disclose all information available and to fully cooperate with all investigations into the killing.

Canada's envoy said: "The explanations provided by Saudi Arabia to date are insufficient."

Hatice Cengiz, a Turkish writer and Khashoggi's fiancée, told the council: "I want to know who gave the order to kill Jamal and who else knew. I want to know where is his body."

"Those who are behind the murder and cover-up should face punishment," she said.