US President Donald Trump has denied covering up for ally Saudi Arabia in the suspected murder of a critical journalist, saying that he should know what happened to Jamal Khashoggi within days.

"No not at all, I just want to find out what's happening," Mr Trump told reporters in the White House when asked if his cautious approach to the scandal amounted to a cover-up.

"I'm not giving cover at all."

Mr Khashoggi was last seen at the Saudi consulate in Istanbul on 2 October.

Mr Trump said he would get a "full report" from Secretary of State Mike Pompeo on his return from meetings with Saudi leaders and then know the truth of what occurred.

"We will probably know that by the end of the week," Mr Trump said.

The US President has appeared to be on the defensive since Turkish sources first accused Saudi agents of killing Mr Khashoggi, a US resident and Washington Post contributor, inside the Saudi consulate in Istanbul.

Mr Trump says that Saudi Arabia is valued as a historic customer for the US weapons industry. Earlier today, he told Fox Business that the US also relies on the kingdom in the fight against terrorism.

Mr Khashoggi, a former regime insider who became critical of Crown Prince Mohammed bin Salman, has not been seen since he stepped inside the Saudi consulate to sort out marriage paperwork.

The controversy has blown a massive hole in attempts by Prince Mohammed to promote himself as a modern reformer and led to a spate of cancelations from a major Riyadh investment conference scheduled next week.

Khashoggi disappearance: two weeks on

International Monetary Fund chief Christine Lagarde has postponed a planned trip to the Middle East with a stop in Saudi Arabia for an investment conference, following Mr Khashoggi's disappearance. 

"The Managing Director's previously scheduled trip to the Middle East region is being deferred," an IMF spokesman said in a statement, without further explanation.

Ms Lagarde had been set to participate in the kingdom's Future Investment Initiative conference that has been boycotted by global business and media leaders since the suspected death of Mr Khashoggi, who was last seen at the Saudi consulate in Istanbul on 2 October.

On Saturday, during the IMF's annual meetings in Bali, Ms Lagarde had said she would travel to Riyadh for the conference next week.

"Human rights, freedom of information are essential rights and horrifying things have been reported and I am horrified," she told reporters at the time.

"But I have to conduct the business of the IMF in all corners in the world and with many governments.  

"When I visit a country, I always speak my mind... So at this point of time my intention is to not change my plans and to be very attentive to the information that is coming out in the next few days."  

But there have since been growing accusations against top levels of the conservative kingdom over the Khashoggi case, triggering a diplomatic crisis.  

Turkish authorities say the journalist was killed inside the Saudi consulate by a group of hit men travelling from Riyadh, which has insisted without evidence that Mr Khashoggi disappeared after leaving the consulate.

The chief executives of HSBC, Standard Chartered and Credit Suisse have also pulled out of the high profile conference in Saudi Arabia next week after similar decisions by JP Morgan & Chase Co chief executive Jamie Dimon and BlackRock chief executive Larry Fink.