Comments by the UN human rights body comes amid the growing chorus of concern over the fate of the Saudi journalist.

The United Nations human rights office has called on Turkey and Saudi Arabia to properly investigate the fate of prominent Saudi journalist and regime critic Jamal Khashoggi, who vanished after entering the Saudi consulate in Istanbul last week.

“If reports of his death and the extraordinary circumstances leading up to are true, this is truly shocking,” spokeswoman Ravina Shamdasani said during a press briefing in Geneva, Switzerland, on Tuesday.

Shamdasani’s comments come shortly after US President Donald Trump, Vice President Mike Pence and Secretary of State Mike Pompeo all expressed concern regarding Khashoggi’s whereabouts.

“I am concerned. I don’t like hearing about it. Hopefully, that will sort itself out,” Trump told reporters at the White House on Monday.

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“Right now, nobody knows anything about it. There are some pretty bad stories going around. I do not like it.”

Khashoggi, a US resident, has written articles over the past year critical of Saudi Crown Prince Mohammed bin Salman. On the eve of his planned marriage to a Turkish woman, he entered the consulate on October 2 and has not been seen since.

Turkish officials have said he was murdered inside the building. Riyadh denies that and claims he left the compound on his own.

Meanwhile, on Tuesday, Turkey’s foreign ministry said Saudi Arabia had given permission to search the premises of its Istanbul consulate.

US Vice President Mike Pence also waded into the controversy over the disappeared Saudi, saying “the free world deserves answers”.

Deeply troubled to hear reports about Saudi Arabian journalist Jamal Khashoggi. If true, this is a tragic day. Violence against journalists across the globe is a threat to freedom of the press & human rights. The free world deserves answers. — Vice President Mike Pence (@VP) October 8, 2018

Republican Senator Lindsey Graham said Riyadh must provide “honest answers” about the journalist.

“We agree that if there was any truth to the allegations of wrongdoing by the Saudi government it would be devastating to the US-Saudi relationship and there will be a heavy price to be paid – economically and otherwise,” Graham tweeted.

“Our country’s values should be and must be a cornerstone of our foreign policy with foes and allies alike,” he said.

US Secretary of State Mike Pompeo called for a thorough and open probe by Saudi Arabia.

“We call on the government of Saudi Arabia to support a thorough investigation of Mr Khashoggi’s disappearance and to be transparent about the results of that investigation,” Pompeo said in a statement.

US-based political analyst Bill Schneider told Al Jazeera that the chorus of comments by American officials indicates the level concern over the journalist’s fate, and how it could affect the close relationship between Trump and Saudi leaders.

“They’re under pressure from members of Congress and the press. Members of Congress are talking about an investigation … The result is there is pressure domestically to look into this matter,” said Schneider.

Small pieces of evidence

Reporting from Istanbul, Al Jazeera correspondent Jamal Elshayyal said Turkish authorities have slowly released more information about the case on Tuesday, specifically about 15 Saudi nationals who travelled to Turkey shortly before Khashoggi’s disappearance.

“We understand as we had mentioned from our information that they had come on two different flights. Those two flights then departed – one to Dubai, the other one to Cairo,” Elshayyal said.

“According to the sources, those 15 nationals, among them Saudi security personnel, were divided into two groups. They went to the consulate at the same time that Jamal was there. They were, in fact, waiting for him prior to his arrival,” he said.

“Despite them booking four nights in a hotel near the consulate, none of them spent the night in Turkey. In fact, they all flew back on those private jets, some directly after, or a couple of hours after Jamal entered the consulate. And another group waited for a few hours before they also went to the airport,” Elshayyal added.

Turkish authorities are also examining several vehicles that were on the consulate premises at the time Khashoggi disappeared.

“We understand that there were four vehicles that left the compound of the consulate a couple hours after Jamal had entered and drove directly to the residence of the consul general,” he said.

“[Authorities are] searching for one particular tinted blacked out van that exited the consulate’s building.”

‘Cannot save themselves’

On Monday, Turkish President Recep Tayyip Erdogan said that Saudi officials must prove that Khashoggi left the Saudi consulate in Istanbul.

“We have to get an outcome from this investigation as soon as possible. The consulate officials cannot save themselves by simply saying ‘he has left’,” Erdogan said.

The 59-year-old Khashoggi, a contributor to the Washington Post newspaper, spent the past year in the United States in self-imposed exile after he fled Saudi Arabia amid a crackdown on intellectuals and activists who criticised the policies of bin Salman.

He was last seen by his fiancee, Hatice Cengiz, entering the consulate to obtain a document needed for their marriage. She and Turkish officials say he never emerged, even though Saudi Arabia insists he left the building.

Turkish authorities have said they believe Khashoggi was most likely killed inside the consulate building and his body later removed from the premises, though they have not provided any conclusive evidence so far.

On Tuesday, a picture of a man said to be Khashoggi, entering the Saudi consulate in Istanbul was shared on social media, allegedly showing him entering shortly before his disappearance.

Although the location has been confirmed to be correct, definitive confirmation of the picture to show Khashoggi on the day of his disappearance has not been provided yet.

Can anyone confirm the authenticity of this timestamped picture of CCTV camera showing @JKhashoggi walking into the consulate? Pic is being circulated on WhatsApp groups. pic.twitter.com/mpC8Uh7Dyx — Mohamed Soltan | محمد سلطان (@soltanlife) October 8, 2018

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Openly critical journalist

Khashoggi, 59, has had a long career as a senior journalist in Saudi Arabia and as an adviser to top officials.

But since the emergence of bin Salman, 33, as the centre of power in the kingdom last year, Khashoggi has been openly critical of the monarchy.

He has assailed the prince’s reforms as hollow, accusing him of introducing a new Saudi era of “fear, intimidation, arrests and public shaming”.

Killing someone such as Khashoggi – who long had ties to the royal family and the Saudi intelligence apparatus – in a consulate would be a major escalation in the crown prince’s rise.

Ties between Turkey and Saudi Arabia are at a low point over Ankara’s support for Qatar last year in its dispute with Saudi Arabia and other Gulf nations. Turkey sent food to Qatar and deployed troops at its military base there.

Saudi Arabia is also annoyed by Ankara’s rapprochement with its regional rival, Iran.