The CIA has reportedly concluded that Saudi Crown Prince Mohammad bin Salman ordered the assassination of journalist Jamal Khashoggi in Istanbul – a finding that contradicts Saudi government assertions that he was not involved.

American officials have expressed high confidence in the agency’s assessment, which is the most definitive to date, allegedly linking Saudi Arabia's crown prince to the killing, according to The Washington Post, which originally reported the story.

The CIA found that 15 Saudi agents flew on government planes to Istanbul and carried out the killing at the Saudi consulate, the report said.

The White House declined to comment on the matter, although such a conclusion would likely complicate efforts by Donald Trump's administration to preserve US ties with one of the closest American allies in the region.

The CIA’s conclusion about Crown Prince Mohammed’s role was also based on the agency’s assessment of the prince as the country’s de facto ruler who oversees even minor affairs in the kingdom. “The accepted position is that there is no way this happened without him being aware or involved,”the Post said an unnamed US official as saying.

The crown prince has repeatedly denied any involvement or knowledge of the murder of The Washington Post columnist, who regularly criticised both the royal and his regime in the newspaper.

Jamal Khashoggi death: key figures Show all 7 1 /7 Jamal Khashoggi death: key figures Jamal Khashoggi death: key figures Jamal Khashoggi Washington Post journalist who was critical of the Saudi regime and the young Crown Prince Mohammed bin Salman, he was murdered on 2 October in the Saudi Arabian consulate in Istanbul AFP Jamal Khashoggi death: key figures Saudi Crown Prince Mohammed bin Salman Heir to the Saudi throne, Mohammed bin Salman has been implicated in the murder, with US officials claiming that he must have known of the plot AFP/Getty Jamal Khashoggi death: key figures 15 man hit squad Turkish police suspect these 15 men of being involved in the murder of Jamal Khashoggi, issued 10 October, 8 days after the journalist disappeared EPA Jamal Khashoggi death: key figures Saud al-Qahtani Aide to Crown Prince Mohammed bin Salman, Saud al-Qahtani is claimed to have ordered Khashoggi's murder Saud Al-Qahtani/Twitter Jamal Khashoggi death: key figures Maher Abdulaziz Mutreb A former diplomat who often travelled with the Crown Prince, Mutreb was initially claimed to be the leader of the hit squad and is pictured here entering the Saudi consulate on the day of the murder AP Jamal Khashoggi death: key figures Mustafa al-Madani First implicated in the 15 CCTV photos released by the Turkish police, al-Madani was later found to have been used as a body double for Khashoggi, leaving the Saudi consulate dressed in his clothes on the day the journalist was killed CNN Jamal Khashoggi death: key figures Salah bin Jamal Khashoggi (L) Son of the murdered journalist met with Crown Prince Mohammed bin Salman on 23 October EPA

Saudi Arabia has offered a series of sometimes contradictory explanations for Khashoggi’s death on 2 October. After initially denying it played any role in his death, the kingdom then went on to claim he was killed by rogue operatives in its consulate in the Turkish capital of Istanbul.

Earlier this week, Riyadh said it would pursue the death penalty for five suspects charged with ordering and carrying out the killing. The Saudi public prosecutor said that Crown Prince Mohammad had nothing to do with the killing.

The prosecutor said that the murder was carried out by a rogue “negotiation team” – including members of Crown Prince Mohammad's security detail – who had sought to abduct Khashoggi but decided to kill him when he resisted. Khashoggi's body was then dismembered and removed from the consulate.

“After surveying the consulate, the head of the negotiation team concluded that it would not be possible to transfer the victim by force to the safe location in case the negotiations with him to return failed,” said the Saudi prosecutor, according to a report by the country’s official news agency. “The head of the negotiation team decided to murder the victim if the negotiations failed.”

Citing sources familiar with the matter, The Post claims that the CIA came to its conclusion after examining multiple sources of information. Another senior US official reportedly told US news channel CNN, that the conclusion is based on a recording provided by the Turkish government and other evidence.

Among these was a phone call between the crown prince’s brother Khalid bin Salman – the Saudi ambassador to the US – and Khashoggi, during which he reportedly told the journalist that he would be safe to retrieve the marriage documents.

The Washington Post, citing people familiar with the call, said it was not clear if Prince Khalid knew Khashoggi would be killed, but he made the call at his brother’s direction.

The prince swiftly tweeted a denial, shortly after the report was published.

“I never talked to him by phone and certainly never suggested he go to Turkey for any reason. I ask the US government to release any information regarding this claim,” he wrote.

He later published what he claimed was his full response to The Post, in which he said that he had met Khashoggi once in person, in September 2017 and they had communicated via text message after that.

“At no time did Prince Khalid discuss anything related to going to Turkey with Jamal,” he said. “Amb Prince Khalid bin Salman has never had any phone conversations with him.”

He added that people were welcome to check his phone records and mobile phone content to corroborate it.

A spokeswoman for the Saudi embassy in Washington also said in a statement that "the claims in this purported assessment are false.”

She added: “We have and continue to hear various theories without seeing the primary basis for these speculations."

On Thursday, the US Treasury Department having announced on Thursday that 17 Saudi nationals would face sanctions over Khashoggi's killing.

Khashoggi disappearance: Forensic police investigate Saudi consul Show all 12 1 /12 Khashoggi disappearance: Forensic police investigate Saudi consul Khashoggi disappearance: Forensic police investigate Saudi consul According to local media reports, Saudi consul Mohammad al-Otaibi left Turkey on 16 October. A Turkish prosecutor on 15 October has entered the Saudi consulate in Istanbul to investigate the disappearance of dissident Saudi journalist Jamal Khashoggi, an inspection that was being carried out jointly with a Saudi team AFP/Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul Turkish forensic police officers work on the roof of the residence of the Saudi consul in Istanbul EPA Khashoggi disappearance: Forensic police investigate Saudi consul Khashoggi went missing on 2 October when he entered the Saudi consulate to pick up paperwork AP Khashoggi disappearance: Forensic police investigate Saudi consul ISTANBUL, TURKEY - OCTOBER 17: Turkish police arrive to investigate the Saudi Arabian consulate general residence as investigations continue into the disappearance of journalist Jamal Khashoggi on October 17, 2018 in Istanbul, Turkey. Turkish police first entered and searched the Saudi Arabian consulate on October 15 amid a growing international backlash about the disappearance. Khashoggi, a U.S. resident and critic of the Saudi regime, has been missing since visiting the Saudi Arabian consulate on October 2. Turkish officials have said they believe he was killed inside. (Photo by Chris McGrath/Getty Images) ***BESTPIX*** Chris McGrath Khashoggi disappearance: Forensic police investigate Saudi consul Turkish forensic and investigation officers arrive at Saudi Consul's residence on October 17, 2018 in Istanbul. - Saudi Arabia's consul to Istanbul Mohammed al-Otaibion on October 16, 2018 left the Turkish city bound for Riyadh on a scheduled flight, reports said, as Turkey prepared to search his residence in the probe into the disappearance of journalist Jamal Khashoggi. (Photo by OZAN KOSE / AFP)OZAN KOSE/AFP/Getty Images OZAN KOSE AFP/Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul Turkish police arrive to investigate the Saudi Arabian consulate general residence Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul Turkish forensic police search for evidence at the garden of the Saudi Arabia's Consul General Mohammad al-Otaibi AFP/Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul Turkish police search the rooftop of the Saudi Arabian consulate general residence Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul Turkish police arrive at the residence of the Saudi consul General Mohammed al-Otaibi to conduct a search AP Khashoggi disappearance: Forensic police investigate Saudi consul Turkish police officers gather as they prepare to enter Saudi Arabia consul's residence, in Istanbul, Wednesday, Oct. 17, 2018. America's top diplomat is in Turkey, where a strongly pro-government newspaper has published a gruesome recounting of the alleged slaying of Saudi writer Jamal Khashoggi at the Saudi Consulate in Istanbul. (AP Photo/Petros Giannakouris) Petros Giannakouris AP Khashoggi disappearance: Forensic police investigate Saudi consul Turkish forensic police search for evidence at the garden of the Saudi Arabia's Consul General Mohammad al-Otaibi AFP/Getty Images Khashoggi disappearance: Forensic police investigate Saudi consul ISTANBUL, TURKEY - OCTOBER 17: Turkish police search the rooftop of the Saudi Arabian consulate general residence as investigations continue into the disappearance of journalist Jamal Khashoggi on October 17, 2018 in Istanbul, Turkey. Turkish police first entered and searched the Saudi Arabian consulate on October 15 amid a growing international backlash about the disappearance. Khashoggi, a U.S. resident and critic of the Saudi regime, has been missing since visiting the Saudi Arabian consulate on October 2. Turkish officials have said they believe he was killed inside. (Photo by Chris McGrath/Getty Images) Chris McGrath Getty Images

The sanctions come under the Global Magnitsky Act, which relates to actions by foreigners outside of the US that threaten international stability.

“The Saudi officials we are sanctioning were involved in the abhorrent killing of Jamal Khashoggi,” said Treasury Secretary Steven Mnuchin.

“The United States continues to diligently work to ascertain all of the facts and will hold accountable each of those we find responsible in order to achieve justice for Khashoggi’s fiancée, children, and the family he leaves behind,” Mr Mnuchin added.

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The sanctions did not target the Riyadh government, an important US security and economic ally and allowed Mr Trump's administration to stop short of any action that could affect US arms deals with Saudi Arabia worth billions of dollars.

The report also came less than 24 hours after hundreds gathered beneath a grey, rainy sky for a funeral service to mark Mr Khashoggi's passing at a historical mosque in Istanbul, praying before an empty slab of marble where his missing body should have .