"Yes, this is of serious concern, the apparent enforced disappearance of Mr Khashoggi from the Saudi consulate in Istanbul", U.N. human rights spokeswoman Ravina Shamdasani told a Geneva news briefing.
Khashoggi, a contributor to the Washington Post who has been living in the United States since previous year after Saudi Crown Prince Mohammad Bin Salman instituted a crackdown on those critical of his policies, went to the consulate to get paperwork so he could marry his fiancee.
Khashoggi was one journalist who chose to "report the reality".
The details will add to pressure on the Saudi government to explain who the 15 Saudis were and what they were doing during their brief trip to Istanbul. Friends and officials have blamed Riyadh for the dissident journalist's disappearance and alleged murder.
David Rohde, executive editor of The New Yorker tweeted out, "The journalist Jamal Khashoggi had said that the Saudi crown prince, Mohammed bin Salman, has 'no tolerance or willingness to accommodate critics'".
As Khashoggi's disappearance drags on, more countries have been moved to comment on the case.
"I assure you that the reports that suggest that Jamal Khashoggi went missing in the Consulate in Istanbul or that the Kingdom's authorities have detained him or killed him are absolutely false, and baseless", it notes.
Saudi Arabia's Consul General in Istanbul Mohammad Al-Otaibi opened up his mission on Saturday to show that Khashoggi was not on the premises.
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Both he and another man are wanted for attempted murder for a Novichok attack on Sergei Skripal and his daughter Yulia in March. Dawn Sturgess , 44, died in June after being exposed to the same weapons-grade nerve agent after it was apparently discarded.
Turkish President Tayyip Erdogan, whose AK Party is rooted in political Islam, also supported a government in Egypt led by the Muslim Brotherhood, which Saudi Arabia has designated a terrorist movement.
On Sunday, another official assessed as "high probability" that Khashoggi was killed in the consulate and his body was taken away.
"The consulate officials can not save themselves by simply saying "he has left".
He had been living since past year in the United States, in a self-imposed exile. Saudi cooperation is important for Turkey in Syria.
Yasin Aktay, who advises Turkish President Tayyip Erdogan in his ruling AK Party, told Reuters that Turkish authorities believed a group of 15 Saudi nationals were "most certainly involved" in the matter, and added that statements by Saudi officials on the absence of camera records were not honest. "We are aware of the latest reports and are working urgently to establish the facts, including with the government of Saudi Arabia".
Turkey has an extensive system of motorway cameras that are regularly used to provide evidence in criminal probes.
Khashoggi was right about Saudi repression.
Reporters Without Borders (RSF) said on Twitter that if reports of his death were confirmed, "this would constitute a horrific, utterly deplorable, and absolutely unacceptable assault on press freedom".
Erdogan's adviser Aktay told Reuters that Turkish authorities believed the group of 15 Saudi nationals were "most certainly involved" in his disappearance.