The United Nations human rights office and the US have 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, yesterday.

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.

"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.

State Department senior officials

have spoken with Saudi Arabia through diplomatic channels about the matter, the top US diplomat added.

Pompeo's statement came after Trump earlier on Monday told reporters at the White House: "I am concerned. I don't like hearing about it.

Hopefully that will sort itself out."

The issue threatens to strain the close relationship Prince Mohammed has forged with the Trump administration, which until now has been willing to turn a blind eye to alleged Saudi human rights violations in Yemen, where it leads a coalition bombing Houthi rebels that has killed thousands of civilians.

Trump has instead focused on US and Saudi shared interests in ratcheting up pressure on Iran.

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, reported Aljazeera Online.

" Saudi authorities said they were open to cooperation and that a search can be conducted at the consulate building," the Turkish foreign ministry spokesman Hami Aksoy said in a statement The search will take place as part of the official investigation, which was being conducted "in an intense manner", he said without giving any date.

Turkish police were looking into two private aircraft which landed at Istanbul's Ataturk airport yesterday at different times carrying 15 people of interest in the case, as well as the possibility that Khashoggi was kidnapped and taken aboard one of the planes, local media reported.