US President Donald Trump on Wednesday demanded Saudi Arabia provide answers over the disappearance of journalist and US resident Jamal Khashoggi, whom Turkish officials suspect was murdered after entering the Saudi consulate in Istanbul.

The Trump administration sharply upped the pressure, reversing an initially low-key response after Washington Post contributor Khashoggi vanished on October 2.

Trump said he had talked "more than once" and "at the highest levels" to partners in Saudi Arabia, which is one of Washington's closest allies and a key market for the US weapons industry.

"We're demanding everything," Trump told reporters. "We cannot let this happen, to reporters, to anybody."

"We are very disappointed to see what's going on. We don't like it and we're going to get to the bottom of it," he added.

Twenty-two senators wrote to Trump invoking the Global Magnitsky Human Rights Accountability Act, which requires the president to open an investigation and determine whether sanctions should be imposed.

The act is used in cases of suspected "extrajudicial killing, torture, or other gross violation of internationally recognized human rights against an individual exercising freedom of expression," the senators said.

Trump spokeswoman Sarah Sanders said National Security Advisor John Bolton, Secretary of State Mike Pompeo and Trump's close aide and son-in-law Jared Kushner had all spoken to Crown Prince Mohammed bin Salman over the past two days.

The United States has not confirmed Turkish claims that Khashoggi, a US resident and one of the more outspoken critics of the regime of King Salman and his son Prince Mohammed, had been lured to the Istanbul consulate and murdered by a team of 15 government operatives sent by Riyadh to Istanbul.

But The Washington Post reported Wednesday that Prince Mohammed himself had ordered the operation targeting Khashoggi.

Saudi officials were heard discussing a plan to lure Khashoggi from the US state of Virginia, where he resided, and detain him, the newspaper said, citing unnamed US officials discussing intelligence intercepts.

The case has sparked outrage from human rights and journalism groups.

In the calls by Bolton, Kushner and Pompeo, Sanders said, "they asked for more details and for the Saudi government to be transparent in the investigation process."

Trump also said he was looking into a meeting in the White House with Khashoggi's fiancee Hatice Cengiz.

The Washington Post, where Khashoggi has been a regular contributor over the past year, also called for answers.

"Reports about Jamal's fate have suggested he was a victim of state-sponsored, cold-blooded murder," said Post publisher and chief executive Fred Ryan.

"Silence, denials and delays are not acceptable. We demand to know the truth," he added.

Khashoggi, 59, is a longtime leading Saudi journalist and former government advisor who went into exile last year after 33-year-old Prince Mohammed rose to power under his father the king.

He has been critical of the monarchy's continued arrest of critics on both the left and right, despite its professed reforms.

He has also repeatedly assailed Riyadh's role leading the war against Yemen's Huthi rebels, a campaign closely identified with Prince Mohammed that has resulted in thousands of civilian deaths and generated a major humanitarian disaster.

Khashoggi was last seen entering the Saudi consulate in Istanbul to obtain papers for his pending marriage to Cengiz, who is Turkish.

In his last interview three days before his disappearance, he said he did not think he would return to Saudi Arabia.

"When I hear of the arrest of a friend who did nothing that (deserved being) arrested, it makes me feel I shouldn't go," he told the BBC. (AFP)

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