In a harsh statement on Wednesday, Agnes Callamard, UN special rapporteur on extrajudicial executions, said ignoring the targeted killing will put the protection of free speech at risk.
"The time to act is now," she said, reiterating her appeal for an international criminal investigation.
Callamard criticised the UN while presenting her report into the killing of Khashoggi, who was murdered by a team of Saudi agents in the country's consulate in Istanbul, to the Human Rights Council in Geneva.
Addressing the media on Tuesday she had expressed her frustration at the UN, saying "there was a role for the UN to investigate this. The silence of this intergovernmental body and the lack of measures were a disservice to the UN and to the world."
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Secretary-General Guterres gave a chilly response to Callamard's appeal.
"The only way to effectively pursue such an investigation is through a Security Council's resolution under the appropriate measures," said Guterres's spokesman Stephane Dujarric on Tuesday, adding that the secretary-general was "not hiding behind any protocol".
Callamard on Wednesday told reporters she was "disappointed" by the response, adding that "it is in his power to ... to establish this panel of [criminal] experts."
"I don't believe you need a member state official demand," she added. "This being said, I certainly call on member states to proceed with that official demarche so that the secretary-general will not be able to just create more firewalls between him and his responsibilities to take action."
Callamard is an independent expert who does not speak for the UN, but reports her findings to it.
Saudi Arabia said it will reject any attempt to take the inquiry into the killing of Khashoggi away from its judiciary, accusing Callamard of "breaching UN procedures".
Addressing member states at the UN Human Rights Council on Wednesday, Saudi Ambassador Abdulaziz al-Wasil said Callamard's report was full of "prefabricated ideas".
"We reject any idea to remove the inquiry from our national justice system," said al-Wasil after Callamard suggested the launch of an international criminal investigation built on her findings.
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Her 100-page report released last week linked the killing of Khashoggi to the kingdom's highest authorities.
On Wednesday, she strongly reiterated how her human rights inquiry pointed presumably to the involvement of Crown Prince Mohammed bin Salman (MBS) and his close aid Saud al-Qahtani in the murder and disappearance of Khashoggi.
"Fingers have been pointed, supported by non-credible sources … [Callamard] has breached her mandate, she has not respected our national laws," said al-Wasil.
The Turkish delegate said Ankara was ready to continue its cooperation with the UN in the case.
Saudi Arabia is conducting a trial of 11 Saudi nationals apparently involved in the killing of the former Washington Post columnist, who lived in self-exile in the United States and was in Turkey to obtain documents for his upcoming marriage.
Riyadh has denied Callamard access to the country and has refused to collaborate with the inquiry she opened in January.
In response to al-Wasil's comments, Callamard said the ongoing trial in Saudi Arabia was not carried out in "good faith" and was not conducted in cooperation with Turkey. "Overall it does not meet international standards," she said.
Only 11 out of 15 operatives who were at the consulate are being indicted, but their names and their charges have not been made public, said Callamard.
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She said the chain of command's responsibility was not being investigated and the trial's proceedings were contradicting the statements made by the public prosecutor.
"For those who want to see a proper criminal trial taking place, an official request should be undertaken by the UN secretary-general upon the request of at least one member state," she said.
Saudi allies come to the rescue
Some Saudi allies, including the United Arab Emirates (UAE), Bahrain and Egypt, criticised the report and issued statements in support of Riyadh.
Emirati Ambassador Obaid al Zaabi said the "hasty conclusions" of the report showed it was based on "unreliable information and media articles".
The Emirati delegate said the "fair trials" under way in Saudi Arabia showed that the kingdom had taken a "serious attitude" towards the case.
Russia criticised Callamard for going "beyond her mandate".
European countries spoke in favour of the report and backed an international criminal investigation.
The European Union delegate said the bloc endorsed Callamard's proposal that full accountability could be obtained "only with a prompt, thorough and transparent international investigation."
Norway, on behalf of Nordic and Baltic countries, Australia, Finland, and the Netherlands, expressed strong concerns about the growing pattern of extrajudicial killings of journalists and dissidents.
Some asked for clarifications over the initiation of an ad hoc body to pursue the investigation. Endorsing the EU statement, Germany and Italy both acknowledged the need for an international investigation.
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Addressing the Council after the reactions of member states, Khashoggi's fiancee Hatice Cengiz called on Guterres to engage member states to come up with the international investigation.
"Truth always wins and history will remember who stood by the truth and who did not," Cengiz said.
Rodney Dixon QC, an expert in international law and human rights, said the UN itself could have a full investigation and establish a court to investigate and prosecute this matter.
"If Saudi Arabia is not going to investigate and prosecute this matter fully then it must go to an international body which must be established for that purpose," said Dixon.
"States have universal jurisdiction over very serious international crimes hence if a country like Saudi Arabia does not act then other states should, that is really the trust of the report's recommendations."
But for a full investigation and prosecution to take place, there needs to be the political will among member states, said Dixon.