The United States should "demand answers" from Saudi Arabia about the disappearance and alleged killing of journalist Jamal Khashoggi, The Washington Post said late Sunday — and punish the kingdom if cooperation is lacking.

Khashoggi, a contributor to the Post who has been critical of Saudi Crown Prince Mohammed bin Salman, vanished after an appointment with Saudi officials on Tuesday at the consulate in Istanbul.

A Turkish government source has said that police believe the journalist was murdered — a claim denied by Riyadh.

"The United States must now make a concerted effort to determine all the facts about Mr Khashoggi's disappearance," the Post said in an editorial, imploring Washington to "demand answers, loud and clear."

Read: ‘This has not been business as usual in my country’: excerpts from Khashoggi’s writings

Noting that President Donald Trump has treated Prince Mohammed as a "favored ally", the newspaper said the kingdom now should reciprocate with information about Khashoggi's whereabouts.

"If the crown prince does not respond with full cooperation, Congress must, as a first step, suspend all military cooperation with the kingdom," the Post said.

The newspaper called on Turkey to reveal any evidence that it has about Khashoggi's alleged murder, and to "spare no avenue to investigate".

It also said that Riyadh should explain the presence of about 15 Saudi nationals, some of them officials, who traveled to Istanbul and were at the consulate at the same time as Khashoggi.

"We are hoping against hope that Mr Khashoggi is unharmed and will soon return to his writing desk," the Post said.

"If the reports of his murder prove true, grief must be accompanied by accountability for those who carried out the murder and those who ordered it."

Khashoggi, 59, is a former government advisor who has criticised some of Prince Mohammed's policies and Riyadh's intervention in the war in Yemen.

He has lived in the United States since last year to avoid possible arrest, and has written a series of columns for the Post on Saudi affairs.