A member of Saudi Crown Prince Mohammed bin Salman's entourage during several trips abroad walked into the Saudi Consulate in Istanbul just before writer Jamal Khashoggi vanished there, a surveillance photo leaked on Thursday shows, drawing the kingdom's heir-apparent closer to the columnist's alleged slaying.

The man, identified by Turkish officials as Maher Abdulaziz Mutreb, has been photographed in the background of Prince Mohammed's trips to the US, France and Spain this year.

Turkish officials say he flew into Istanbul on a private jet along with an “autopsy expert” October 2 and left that night. That was the same day Khashoggi, a columnist for The Washington Post who wrote critically of Prince Mohammed's rise to power, entered the consulate and was not seen again.

Saudi Arabia, which initially called the allegations “baseless”, has not responded to repeated requests for comment from The Associated Press over recent days, including on Thursday over Mutreb's identification. The AP could not immediately reach Mutreb for comment.

But Mutreb's appearance at the consulate, as well as later at the consul general's residence, adds to the growing pressure on Saudi Arabia amid international outrage over the disappearance of the writer, whom Turkish officials say was killed and dismembered.

In a further sign of that pressure, US Treasury Secretary Steven Mnuchin said he will not attend an investment conference in Saudi Arabia, as did senior government officials from France, Britain and the Netherlands. Several top business executives have also canceled plans to attend, as has the head of the International Monetary Fund, Christine Lagarde.

Analysts say that as long as the Saudis refuse to acknowledge what happened to Khashoggi, the leaks about the case will probably continue.

“Turkey wants to show to the world that it cannot be ignoble, selling values and principles in political deals with US or Saudi to try to bury the truth and come up with an acceptable scenario,” said Yusuf Katipoglu, a Turkish analyst.

The pro-government Sabah newspaper on Thursday first published the images of Mutreb, showing him walking past police barricades at the consulate at 9:55am with several men trailing behind him. Khashoggi arrived at the consulate several hours later at 1:14pm, then disappeared while his fiance waited outside for him.

A report on Wednesday by the pro-government newspaper Yeni Safak, citing what it described as an audio recording of Khashoggi's slaying, said a Saudi team immediately accosted the 60-year-old journalist after he entered the consulate, cutting off his fingers and later decapitating him.

Previously leaked surveillance footage showed consular vehicles moving from the consulate to the consul general's official residence, some 2 kilometres (1.2 miles) away, a little under two hours after Khashoggi walked inside.

The Sabah-published pictured showed an image of the Mutreb at 4:53pm at the consul's home, then at 5:15pm checking out of a hotel. He later cleared an airport security check at 5:58pm before flying out of Istanbul.

Mutreb's name matches that of a first secretary who once served as a diplomat at the Saudi Embassy in London, according to a 2007 list compiled by the British Foreign Office. The same name also appears in an email published by WikiLeaks from the 2015 breach of surveillance company Hacking Team of Saudi officials being trained to use their software.

That breach showed how governments were increasingly turning to mercenary hackers-for-hire to pry into the cellphones and computers of their domestic opponents.

Mutreb's identity was confirmed by Turkish officials, who spoke on condition of anonymity because the investigation was ongoing. Mutreb also was identified in state and pro-government media reports.

It's unclear what relationship Mutreb has with Prince Mohammed.

Images shot by the Houston Chronicle and later distributed by the AP show Mutreb in Prince Mohammed's entourage when he visited a Houston subdivision in April to see rebuilding efforts after Hurricane Harvey. The same man wore lapel pins, including one of the US and Saudi flags intertwined, that other bodyguards accompanying Prince Mohammed wore on the trip.

The three-week trip across the US saw Prince Mohammed meet with business leaders and celebrities, including Amazon billionaire Jeff Bezos, who now owns the Post.

Mutreb also has appeared in images on Prince Mohammed's trips to Boston, as well as Spain and France.

The Sabah report came as Turkish crime-scene investigators finished an overnight search of both the consul general's residence and a second search of the consulate itself. Authorities have not said specifically what they found, although technicians carried out bags and boxes from the consul general's home. He left Turkey on Tuesday.

The searches and the leaks in Turkish media have ensured attention remains focused on what happened to Khashoggi, a Washington Post columnist who went into a self-imposed exile in the US after the rise of Prince Mohammed. It also put further strains on the relationship between the kingdom, the world's largest oil exporter, and its main security guarantor, the US, as tensions with Iran and elsewhere in the Middle East remain high.

President Donald Trump, who initially came out hard on the Saudis over the disappearance but since has backed off, said Wednesday that the US wanted Turkey to turn over any audio or video recording it had of Khashoggi's alleged killing “if it exists.”

After briefing Trump on Thursday on his talks this week with leaders in Saudi Arabia and Turkey, US Secretary of State Mike Pompeo said he told the Saudi rulers that the US takes “very seriously” the disappearance of Khashoggi and will await the outcome of investigations by the kingdom and Turkey before deciding how to respond.

Meanwhile, the Committee to Protect Journalists, Human Rights Watch, Amnesty International, and Reporters Without Borders jointly called for a UN investigation of the Khashoggi disappearance.

“If the government of Saudi Arabia is not involved in Jamal Khashoggi's fate, it has the most to gain in seeing an impartial UN investigation determine what happened,” said Sherine Tadros of Amnesty International.

“Without a credible UN inquiry, there will always be a cloud of suspicion hanging over Saudi Arabia, no matter what its leadership says to explain away how Khashoggi vanished.”

The Post published on Thursday what it described as Khashoggi's last column, in which he pointed to the muted international response to ongoing abuses against journalists by governments in the Middle East.

“As a result, Arab governments have been given free rein to continue silencing the media at an increasing rate,” Khashoggi wrote. He added: “The Arab world is facing its own version of an Iron Curtain, imposed not by external actors but through domestic forces vying for power.”