Trump says Saudi killing of Khashoggi 'worst cover-up in history'

US President Donald Trump charged on Tuesday that a Saudi operation to kill journalist Jamal Khashoggi in Riyadh's consulate in Istanbul had given rise to "one of the worst cover-ups" in history.

"They had a very bad original concept. It was carried out poorly and the cover-up was one of the worst in the history of cover-ups," Trump declared in the Oval Office.

"Bad deal, should have never been thought of. Somebody really messed up. And they had the worst cover-up ever," he added.

"Because whoever thought of that idea, I think is in big trouble. And they should be in big trouble. Okay?"

After more than two weeks of near silence, Saudi Arabia admitted on Saturday that Khashoggi, 59, was killed in its Istanbul consulate on 2 October, in what it said was an operation gone wrong.

But Turkey's President Recep Tayyip Erdogan said on Tuesday that Khashoggi was targeted in a meticulously planned killing - and demanded that 18 suspects detained by Saudi Arabia be extradited to Istanbul to face trial.

Asked about Erdogan's call for those behind Khashoggi's "savage murder" to face punishment, Trump said the Turkish president had been "pretty rough on Saudi Arabia."

But when questioned about whether he believes the Saudi account of events leading up to Khashoggi's death, Trump replied, "I want to see the facts first" - while reiterating Riyadh's importance as a US ally in the Middle East.

"Look, Saudi Arabia has been a really great ally. They've been one of the biggest investors, maybe the biggest investor, in our country," he said.

Earlier, US Vice President Mike Pence vowed to press Saudi Arabia for answers over journalist Jamal Khashoggi's "brutal murder".

Top US senators have scolded Riyadh for the killing of Khashoggi, pledging that Congress would take decisive action against Saudi Arabia

A former royal family insider turned critic of the Saudi crown prince, Khashoggi disappeared after he entered the consulate to collect a document for his upcoming marriage.

The murder saga has provoked outrage across the globe, tarnishing Saudi Crown Prince Mohammed bin Salman's hard-fought image as a "reformer", and threatening to damage the kingdom's close ties with the US and Europe.

Several high-profile sponsors and speakers withdrew from a top economic summit in Riyadh this week, including the leaders of the World Bank and the International Monetary Fund, as well as JP Morgan, Uber and Ford.

Major Western media outlets including Bloomberg, CNN, the Financial Times and The Economist also pulled out of covering the summit.