In the UN report, Agnes Callamard accused Saudi Arabia of a 'deliberate, premeditated execution' but made no conclusion as to the guilt.
Saudi dissident, Jamal Khashoggi's fiancee called for an international investigation on the journalist's killing on Tuesday after United Nations released a report on the case in which it held Saudi Arabia responsible for his' killing.
After a six-month investigation, the 100-page report by the UN special rapporteur on extrajudicial executions was released on Monday which held Saudi Arabi Crown Prince Mohammed Bin Salman and other Saudi officials liable for the murder.
In the UN report, Agnes Callamard accused Saudi Arabia of a "deliberate, premeditated execution" but made no conclusion as to the guilt.
Speaking at an event in Switzerland's Geneva, Hatice Cengiz said the Saudi government had eliminated evidence, hence rendering its investigation invalid.
"In this matter, one of the points indicated by the report is that there are evidence that Saudi Arabia, Saudi Arabian administration seem to have eliminated some pieces of evidence during the investigation process," she said.
"Within that scope, the investigation in Saudi Arabia is not legitimate anymore," Cengiz said, adding, "So the necessity for an international investigation about this murder arises again because there are findings in the report that Saudi Arabia had attempted to eliminate or get rid of the pieces of evidence by Saudi Arabia."
Agnes Callamard who was also speaking at the event said, the killing violated key principles of international relations making it an "international crime," hence the UN and the international community must be able to hold a fair investigation and trial.
"I don't think the world can afford to turn a blind eye or to turn the page on a killing such that of Mr Khashoggi that had taken place in a foreign country, it violated key principles of international relations."
"It is in my view an international crime. For that reason, the international community, the United Nations must be able to investigate, analyse and report to it fairly, honestly and be prepared to take the actions in response to it."
Meanwhile, Saudi foreign minister Adel al-Jubeir rejected the UN report, and cast doubt on its credibility, saying it contained "clear contradictions and baseless allegations".
Jubeir also rejected any attempt to influence the trial or remove it from the Saudi judicial process.
Jamal Khashoggi, who worked as a journalist for The Washington Post, was killed in October last year in Saudi Arabia's consulate in Turkey. A critic of a Saudi prince, Mohammed Bin Salman, Khashoggi was last seen at the Saudi consulate in Istanbul on October 2 where he was to receive papers ahead of his wedding.