In his toughest comments yet, Trump told the Wall Street Journal he wanted to believe Crown Prince Mohammed bin Salman when he said that lower level officials were to blame for the killing at the Saudi mission.

But he suggested responsibility lay higher up: "Well, the prince is running things over there more so at this stage. He's running things and so if anybody were going to be, it would be him."

His comments heaped pressure on his close ally amid a global outcry over the journalist's death, and came hours before Prince Mohammed's appearance at a Saudi investment conference where he is due to make his most high profile comments since Khashoggi was killed on Oct. 2.

A number of high profile business and political figures have pulled out of the conference over the death of the journalist, a prominent critic of Saudi Arabia's de facto ruler.

Turkish President Tayyip Erdogan spoke to Prince Mohammed on Wednesday, and the two discussed the steps needed to bring to light all aspects of the killing of Khashoggi, a presidential source said.

An adviser to Turkey's president said Prince Mohammed had "blood on his hands" over Khashoggi, the bluntest language yet from someone linked to Tayyip Erdogan.

Saudi authorities did not immediately respond to a request for comment about the remarks by Trump and the Erdogan adviser.

Riyadh has blamed a "rogue operation" for the death of the prominent Saudi journalist and said the crown prince had no knowledge of the killing.

The death of Khashoggi, a US resident and Washington Post columnist, has sparked global outrage and threatened relations between Riyadh and Washington as well as other Western nations.

For Saudi Arabia’s allies, the burning question has been whether they believe that Prince Mohammed, who has painted himself as a reformer, has any culpability in the killing, a possibility raised by several US lawmakers.

CONFLICTING ACCOUNTS

In an internal email sent widely to US State Department employees, Secretary of State Mike Pompeo said the United States wants perfect clarity on exactly what happened in the death of Khashoggi.

Saudi Arabia has given conflicting accounts about Khashoggi's killing. It first denied his death and later said Khashoggi - a US resident and columnist for the Washington Post - died inside the consulate after a fight.

On Sunday Riyadh called the killing a "huge and grave mistake," but sought to shield the crown prince from the widening crisis, saying he had not been aware.

Turkish security sources say that when Khashoggi entered the consulate, he was seized by 15 Saudi intelligence operatives who had flown in on two jets just hours before.

"It is a disgrace that reaches all the way to Crown Prince (Mohammed bin) Salman. At least five members of the execution team are (Mohammed bin) Salman's right hands and are people that wouldn't act without his knowledge," Ilnur Cevik, one of several advisers to Turkey's Erdogan, wrote in the Yeni Birlik newspaper.

"Even if US President Trump saves (Mohammed bin) Salman, in the eyes of the world he is a questionable person with Khashoggi's blood on his hands," Cevik's column said.

It was not immediately clear if Cevik's remarks reflected the views of Erdogan, who in speeches on Tuesday and Wednesday highlighted the need for all of those responsible - "from those who ordered it to those who carried it out" - to face justice.

Saudi Arabia has agreed to a joint investigation into Khashoggi's killing with Turkey, allowing a joint search of its consulate in Istanbul days after the journalist's death.

As part of that probe, Turkish police in Istanbul have been granted permission to search a well in the garden of the Saudi consulate, broadcaster NTV said on Wednesday. Earlier, Saudi officials had refused to sanction a search.

SAUDI RESPONSE

Saudi Arabia has detained 18 people and dismissed five senior government officials as part of the investigation. Among those fired was Saud al-Qahtani, a top aide to Prince Mohammed. According to two intelligence sources, Qahtani ran Khashoggi's killing by giving orders over Skype. (Full Story)

Khashoggi's death and ensuing uproar have shaken global confidence in ties with the world's top oil exporter.

In the latest sign of unease, Swiss Foreign Minister Ignazio Cassis said the killing pointed clearly to a violation of human rights that required a review of Switzerland's ties to Riyadh.

"The clues that are emerging centimetre by centimetre speak a clear language: A violation of human rights and the rule of law," he told Swiss tabloid Blick.

"We have to ask ourselves the question of what that means for our bilateral relations. And we will definitely do that."

The German government is discussing how to deal with arms exports to Saudi Arabia that have already been approved but not yet delivered, government spokesman Steffen Seibert said.

France will take appropriate measures if Saudi Arabia's guilt over the killing of Khashoggi is clearly established, government spokesman Benjamin Griveaux said.