US President Donald Trump backed the embattled Saudi monarchy on Tuesday, saying that Crown Prince Mohammed bin Salman had told him he had no idea what happened to missing Saudi journalist Jamal Khashoggi.

The prince "totally denied any knowledge of what took place in their Turkish Consulate," Trump wrote on Twitter, after speaking with the 33-year-old who effectively rules the oil-rich kingdom.

"He was with [US} Secretary of State Mike Pompeo during the call, and told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly," Trump continued.

His comments came after Turkish investigators searched Saudi Arabia's consulate and official residence in Istanbul, looking for clues to the fate of Khashoggi, who was last seen entering the consulate on October 2.

The White House is facing mounting pressure to act, including calls from within Trump's own centre-right Republican Party for sanctions, as anger grows about the journalist's disappearance.

Turkish officials have told media that Khashoggi, a sharp critic of the crown prince and who wrote a column for the Washington Post, was murdered inside the consulate. Saudi Arabia has denied the allegations.

Pompeo held talks with the prince - who only recently was hailed in some policy circles as a reformer - and other officials on Tuesday, after being dispatched to Riyadh by Trump a day earlier.

Photos released from his meeting with the crown prince showed them smiling and shaking hands.

"During each of today’s meetings the Saudi leadership strongly denied any knowledge of what took place in their consulate in Istanbul," Pompeo said in a statement afterwards.

"My assessment from these meetings is that there is serious commitment to determine all the facts and ensure accountability, including accountability for Saudi Arabia’s senior leaders or senior officials," he said.

Following his Riyadh visit, Pompeo will be in Turkey on Wednesday to meet Foreign Minister Mevlut Cavusoglu.

Meanwhile, the G7 foreign ministers - representing the US, Britain, France, Japan, Germany, Canada and Italy - and the EU's foreign policy chief issued a joint statement saying those responsible for Khashoggi's disappearance "must be held to account."

IMF Managing Director Christine Lagarde also postponed a trip to the Middle East which was to have included a stop at the October 23-25 Future Investment Initiative in Saudi Arabia.

A spokesperson made the announcement without specifying the reason for the delay. However, Lagarde is only the latest high-profile attendee to pull out of the conference following Khashoggi's disappearance.

JP Morgan chief executive Jamie Morgan, Ford Motors chairman Bill Ford and Uber boss Dara Khosrowshahi have also pulled out, as well as major media sponsors including CNN, CNBC, the Financial Times and the New York Times.

Saudi Arabia has been unable to explain what happened to Khashoggi, other than simply saying that he left the consulate, though there is no evidence that this happened.

Turkish police units searched the consul's residence in Istanbul, entering the building shortly after the top diplomat at the station, Mohammad al-Otaibi, left the country on a commercial flight to Saudi Arabia, according to Turkish state broadcaster TRT.

The search at the residence came hours after Turkish and Saudi teams wrapped up a nine-hour search of the consulate in the early hours of Tuesday, according to state news agency Anadolu.

It was the first time Turkish officials had been allowed into the buildings.

Turkish President Recep Tayyip Erdogan said Tuesday investigators were looking into possible traces of "toxic materials" that had been painted over, local media reported.

The Washington Post issued a statement saying the Saudi government "can no longer remain silent" and that Riyadh owes the Khashoggi family an explanation. The media outlet said "it cannot be business as usual" with the Saudi government until answers emerge.

Trump had previously promised "severe punishment" if the kingdom's leadership was responsible for the death of Khashoggi.

At the same time, he has made clear he does not want to cancel massive arm sales to Saudi Arabia, saying it would hurt US manufacturers and jobs.

Top oil exporter Saudi Arabia has vowed to respond to steps taken against it with "greater action" and warned of consequences for the global economy.

Khashoggi, 60, was once an insider with some Islamist sympathies who only recently emerged recently as a critic of the prince's policies, of Riyadh's role in the war in brutal Yemen and the "repressive" tactics used by the Saudi government against opponents at home.

He has lived in self-imposed exile in the United States since last year "over concerns that he would be arrested or prevented from travelling" by Saudi Arabia, the Washington Post said.