All smiles: Saudi Crown Prince Mohammed bin Salman (centre) posing for a selfie during the Future Investment Initiative in Riyadh. Photo: Getty Images

US President Donald Trump, in his toughest comments yet, said Saudi Arabia's crown prince bore ultimate responsibility for the operation that led to Jamal Khashoggi's killing, piling pressure on his ally amid a global outcry over the journalist's death.

The 'Wall Street Journal' published Mr Trump's remarks hours before Crown Prince Mohammed bin Salman's appearance at a business conference where he is due to make his most high profile comments since Mr Khashoggi was killed on October 2.

An adviser to Turkey's president, meanwhile, said Prince Mohammed had "blood on his hands" over Mr Khashoggi, the bluntest language yet from someone linked to Recep Tayyip Erdogan about the de facto ruler of the kingdom in connection with the death.

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

Mr Trump told the 'Journal' he wanted to believe the prince when he said that lower level officials were to blame for the killing at the Saudi consulate in Istanbul.

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."

The death of Mr 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.

Saudi Arabia has given conflicting accounts about Mr Khashoggi's killing. It first denied his death and later said he 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 Mr Khashoggi entered the consulate, he was seized by 15 Saudi intelligence operatives who had flown in on two private 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 Salman's right hands and are people that wouldn't act without his knowledge," Ilnur Cevik, one of several advisers to Turkey's Mr Erdogan, wrote in the 'Yeni Birlik' newspaper.

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

It was not immediately clear whether Mr Cevik's column reflected the views of Mr 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 conduct a joint investigation into Mr Khashoggi's killing with the Turkish authorities, with its consulate in Istanbul searched 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, after Saudi officials had earlier refused to allow a search.

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, Mr Qahtani ran Mr Khashoggi's killing by giving orders over Skype.

Mr 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.

Irish Independent