Donald Trump has called the response to the killing of Saudi journalist Jamal Khashoggi ”the worst cover-up” in history as the US announced it would sanction Saudi Arabian officials who are implicated in the death.

Mr Trump was speaking hours after Turkish president Recep Tayyip Erdogan dismissed Riyadh’s efforts to blame Mr Khashoggi’s death, within the Saudi consulate in Istanbul, on “rogue operatives”. Mr Erdogan called for Saudi Arabia to search from “top to bottom” to uncover those behind what he called “savage” premeditated murder of Mr Khashoggi – who was last seen alive entering the consulate on 2 October.

Asked in the Oval Office how the killing of Mr Khashoggi could have happened, Mr Trump said: “They had a very bad original concept. It was carried out poorly, and the cover-up was one of the worst in the history of cover-ups.”

Mr Trump said the Khashoggi matter was handled badly by Saudi officials. “Bad deal, should have never been thought of. Somebody really messed up. And they had the worst cover-up ever,” he said. ”Once they thought about it, everything else they did was bad too,” the president added later.

Having spent weeks maintaining that Mr Khashoggi had left the consulate alive, Saudi authorities have said they now believe the writer – a US resident and Washington Post columnist – was unintentionally killed in a “fight” with “suspects” inside the diplomatic premises. That explanation has been met with scepticism, with Turkish officials having alleged that a 15-man ‘hit squad’ was behind the killing and that Saudi officials had planned the operation for days.

The killing has caused global outage, with the fallout straining relations between Riyadh and a number of governments who have called for a transparent investigation. The G7 nations issued a fresh call for “a thorough, credible, transparent and prompt investigation” on Tuesday adding Saudi Arabia needed to put in place “measures to ensure that something like this cannot happen again”.

“There should have never been an execution or a cover-up, because it should have never happened,” Mr Trump said. He added that he had spoken on Monday with Crown Prince Mohammed bin Salman who denied having anything to do with Mr Khashoggi’s killing.

The crown prince has come under pressure over his links to some of the suspects identified as allegedly being involved by Turkish officials. On Tuesday, the Saudi foreign ministry released photos of Mr Khashoggi’s son, Salah bin Jamal Khashoggi, meeting the crown prince and King Salman in Riyadh.

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 crown prince did receive a standing ovation when he appeared at a major investment conference in Riyadh, a conference from which a number of Western government officials and business leaders have withdrawn because of the controversy.

Shortly after Mr Trump made his comments, secretary of state Mike Pompeo said the administration was moving to revoke the visas of Saudi government and intelligence agents suspected of involvement in the death of Mr Khashoggi.

Visa records are confidential and Mr Pompeo did not say precisely who the revocations would affect, but the State Department later said 21 “Saudi suspects” would have visas revoked or would be declared ineligible to enter the US.

Turkish President Erdogan calls on Saudi Arabia to 'bring forward those responsible' for 'savage murder' of Jamal Khashoggi

The penalties are the first action taken by Washington over the affair, with Mr Trump having faced pressure from Congress to sanction Riyadh.

“These penalties will not be the last word on this matter,” Mr Pompeo said at the State Department. The administration “will continue to hold those responsible accountable. We’re making very clear that the United States does not tolerate this kind of ruthless action to silence Mr. Khashoggi, a journalist, with violence”, he said. “Neither the president or I am happy with this situation.”

Mr Pompeo added: “As we continue to develop our understanding of the individuals that were responsible for this, not only those who executed it but those who were connected to it, the world should know that we intend to hold those individuals accountable.” The White House has sent CIA director Gina Haspel to Istanbul to help the Turkish government with its investigation.

Mr Trump’s comments in recent days have ranged from threatening Saudi Arabia with “very severe” consequences and mentioning possible economic sanctions, to more conciliatory remarks highlighting the country’s role as a US ally against Iran and Islamist militants, as well as a major purchaser of American weapons.

Mr Erdogan’s earlier address in Ankara had been highly anticipated, and he sought to hit out at the Saudi version of events. Mr Erdogan did not directly address Crown Prince Mohammed bin Salman but said it was important that any Saudi investigation be carried out “by an impartial and fair delegation” with no connections to the killing.

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“It will not satisfy the public by just pinning this kind of matter on a few security and intelligence officers,” he said. “Covering up this kind of savagery will hurt the conscience of all humanity.”

Mr Erdogan did directly address King Salman, requesting that the 18 Saudi suspects arrested so far be put on trial in Istanbul. There had been speculation that Mr Erdogan might present some of the audio or video evidence that Turkey has allegedly gathered, but the president did not broach the subject.