US Secretary of State Mile Pompeo has called on Saudi Arabia to conduct “thorough investigation” into controversial disappearance of journalist Jamal Khashoggi vanishing after entering the kingdom’s consulate in Istanbul last week.

"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 on Monday.

This comes after the Washington Post urged the US government to "demand answers" from Saudi Arabia about Khashoggi – an opinion writer for the newspaper.

"The United States must now make a concerted effort to determine all the facts about Mr. Khashoggi's disappearance," the newspaper said in an editorial late Sunday.

Khashoggi was known for his critical views on Riyadh. He lived in self-imposed exile in the US since September 2017, when he left Saudi Arabia amid a crackdown on dissident voices.

The New Yorker said in a recent article that the journalist had told it in August that he was worried about his life.

Khashoggi also opposed Riyadh's aggression on Yemen.

The 59-year-old was reported missing last week after entering the Saudi consulate in Istanbul, where he intended to obtain a document necessary to finalize his divorce from his wife. He reportedly never came out of the consulate.

Turkish sources later told the Reuters news agency that the journalist had been killed inside the building.  

An adviser to Turkish President Recep Tayyip Erdogan said on Sunday that he believed Khashoggi was killed at the Saudi consulate.

The reports of his unconfirmed death have sparked outrage. US President Donald Trump on Monday said he was "concerned" about Khashoggi's disappearance, and Turkish President Recep Tayyip Erdogan said he was monitoring the case closely.

Erdogan said Saudi Arabia’s consulate must “prove” that Khashoggi made exit from the mission.

In a Bloomberg interview conducted on Wednesday and published on Friday, Saudi Crown Prince Mohammed bin Salman claimed that Khashoggi “entered” the consulate and “got out after a few minutes or one hour.”

Yemeni activist and 2011 winner of the Nobel Peace Prize, Tawakkol Karman said she believed Khashoggi “was kidnapped in this gangster's den that is supposed to be a consulate.”

Pompeo in his statement on Monday said that the US had received conflicting accounts of Khashoggi's disappearance.

He added that State Department officials have contacted Saudi authorities and are looking into the issue.

KD