The Washington Post, the daily to which Khashoggi was a contributor, added to the mystery by reporting Saudi Crown Prince Mohammed bin Salman had ordered an operation to "lure" the critical journalist back home.

Khashoggi has not been seen since October 2 when he went to the Saudi consulate in Istanbul to obtain official documents for his upcoming marriage. Turkish officials quoted in media have said he was killed but Riyadh denies that.

The mystery has captivated the world but also threatens to harm brittle Turkish-Saudi relations and hurt efforts by the crown prince to improve the image of his country with a reform drive.

Erdogan challenged Saudi Arabia to provide CCTV images to back up its version that Khashoggi had left the consulate safely, indicating he did not find the current Saudi explanations sufficient.

"Is it possible there were no camera systems in a consulate, in an embassy? Is it possible that there was no Saudi camera system where this incident took place?" Erdogan told Turkish reporters in comments published in newspapers.

"If a bird flew, or a fly or a mosquito appeared, the systems would capture this; they (Saudi Arabia) have the most cutting-edge systems," he was quoted as saying.

The consulate said CCTV cameras were not working that day and dismissed the murder claims as "baseless".

- 'Demanding everything' -

The case is also threatening the strong relationship the Trump administration has built with Prince Mohammed, who wants to turn the oil-rich conservative kingdom into a hub for innovation and reform.

The two sides have worked together on challenging Iran despite growing concern over the prince's campaign against dissidents, which critics say has shown up the true face of his rule.

In a reversal from Washington's initial low-key response, Trump demanded answers after he spoke to Saudi authorities "at the highest level". 

"We're demanding everything. We want to see what's going on there," he said. Trump later told "Fox News at Night" that "it would not be a good thing at all" if Saudi involvement was proven.

Khashoggi is a former government adviser who fled Saudi Arabia in September 2017 and lived in the US fearing arrest back home.

In his columns for the Washington Post and comments elsewhere, Khashoggi was critical of some policies of Mohammed bin Salman as well as Riyadh's role in the war in Yemen.

- 'Cannot remain silent' -

While unnamed Turkish officials quoted in the media have been giving sometimes macabre details of the alleged murder, Erdogan has so far been more circumspect.

He has said Saudi Arabia must prove its version of events but so far has stopped short of directly accusing the kingdom or laying the blame on the powerful crown prince.

"It's not possible for us to stay silent regarding an incident like this," Erdogan said.

He added that it would "not be right" to comment yet but said he had "concerns".

Ankara and Riyadh have worked over recent years to maintain cordial relations despite disputes on key issues, such as the ousting of the Islamist Egyptian government and the blockade on Turkey's key regional ally Qatar.

Friends of Khashoggi told the Washington Post that for several months, senior Saudi officials were offering him protection, "even a high-level job working for the government" if the critic returned to the kingdom. Khashoggi was sceptical of such offers.

Following the Washington Post report, State Department deputy spokesman Robert Palladino said the US "had no advanced knowledge of Jamal Khashoggi's disappearance".

Turkish police are looking into a team of 15 Saudis who they say were at the consulate at the same time as Khashoggi and arrived in Istanbul on October 2 on board two private planes. Turkish media have said the 15 were an "assassination team" and that they took the consulate's footage with them.

After images of the men and their names were published by pro-government Sabah daily, media identified most of them as senior figures in Riyadh or close to the crown prince.

Turkish police are also analysing CCTV footage which showed a vehicle that went inside the consulate and then to the consul-general's residence nearby after 1200 GMT, two hours after Khashoggi had entered the mission.

Turkish authorities have been given permission to search the consulate -- Saudi sovereign territory -- but it has not yet taken place.

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