A group of four major US and European human rights and press freedom organisations, alarmed by signs of backroom dealmaking by world powers, called for a United Nations probe into the disappearance and possible murder of Saudi Arabian journalist Jamal Khashoggi, a 60-year-old Washington Post columnist who vanished after entering his nation’s consulate in Istanbul 16 days ago.

The groups, which include Amnesty International, Human Rights Watch, Reporters Without Borders, and the Committee to Protect Journalists, called on Turkey to ask UN Secretary-General Antonio Guterres to launch an investigation to determine who ordered, planned, and carried out what unnamed Turkish and U.S. officials have described to journalists as the extrajudicial killing of Mr Khashoggi.

"Turkey should enlist the UN to initiate a timely, credible, and transparent investigation” Robert Mahoney, deputy executive director of the Committee to Protect Journalists, said in a joint statement released. “UN involvement is the best guarantee against a Saudi whitewash or attempts by other governments to sweep the issue under the carpet to preserve lucrative business ties with Riyadh."

The statement came amid alarm among the rights groups over a recent swirl of backroom diplomatic meetings between top-ranking US, Saudi, and Turkish officials and worries by some that justice for Mr Khashoggi could be undermined by geopolitical machinations. The US needs Saudi oil, Saudi Arabia needs US military supplies and protection, Turkey needs the help and cooperation of both to fix up its ailing economy and protect its southern border from the chaos in neighbouring Syria.

The only chance we have to have an investigation that could possibly not be politicised, is to do it through the UN Sherine Tadros, Amnesty International

"This is exactly why this call is a no-brainer," Sherine Tadros, head of Amnesty International's New York office, told The Independent. "You have the Secretary General of the UN saying we need to know the truth. You have President Trump saying Saudi is being treated as guilty until proven innocent. The only chance we have to have an investigation that could possibly not be politicised, is to do it through the UN."

The UN has been approached about the idea and begun to warm to it, said a source.

Turkish justice minister Abdulhamit Gul told the official Anatolia News Agency Mr Khashoggi’s disappearance was being investigated thoroughly by the Istanbul prosecutors office and insisted that "results are expected to come out soon."

A photo made available by the Turkish Presidential Press Office shows US Secretary of State Mike Pompeo (3-L) during a meeting with Turkish President Recep Tayyip Erdogan (C) and Turkish Foreign Minister Mevlut Cavusoglu (3-R) in Ankara on Wednesday (EPA)

Turkish forensic officers arrive at the Saudi consulate to conduct a new search over the disappearance and alleged slaying of writer Jamal Khashoggi, in Istanbul, early Thursday. (AP)

"Whatever the international law requires will be done," he insisted, calling on press to ignore a torrent of leaks in the media most suspect are being directed by the Turkish leadership to keep pressure on Washington and Riyadh.

Hours earlier, a team of groggy crime scene investigators in hazmat suits could be seen leaving the compound of the Saudi consul general after a nine-hour search of the premises. A team of 15 alleged Saudi operatives apparently jetted into Istanbul from Riyadh hours before Mr Khashoggi’s scheduled October 2 arrival at the consulate and subsequent disappearance, stopping off at the consul general’s residence for two hours before heading back to the airport.

Photos show alleged Saudi operative Maher Mutreb's movements during his visit to Istanbul on day of Jamal Khashoggi's disappearance (SABAH)

The Turkish daily Sabah newspaper published new surveillance camera stills of one of those men, Maher Abdulaziz M. Mutreb, who had been exposed by The New York Times this week as frequent presence in Saudi Crown Prince Mohammed bin Salman’s entourage.

The fresh images showed Mr Mutreb arriving at the consulate shortly before 10am on 2 October, leaving the consul-general’s residence shortly before 5pm, and checking out of his hotel a few minutes later, in the presence of a large suitcase, before being spotted at Istanbul’s Ataturk Airport heading out of the country just before 6pm.

State-directed Saudi press and pro-Saudi social media accounts have described the 15 as “tourists” and has doubled down on its denials, despite reports it might pin the killing on low-ranking officials. “The kingdom is more powerful than enemies’ conspiracies,” read a headline in pro-government Okaz newspaper. "No to threats, no to blackmailing, no to pressure”

Turkish media have also begun to report that some of the vehicles linked to the consulate or the consul-general’s residence have been spotted by surveillance cameras near Istanbul’s massive Belgrade Forest, a large park on the European side of the city, and in a farm in a neighbouring province. A request to Turkish officials to confirm the report yielded no response.

A man holds Yeni Safak newspaper with a headline that reads "(To the Saudi consul) Shut up." (AP)

The steady flow of leaks has kept the Khashoggi affair high on the global news agenda but also added to confusion and possible misinformation about the case. The four rights organisations urged Saudi to waive diplomatic protections and grant access to officials and Turkey to hand over audio and visual evidence to a UN investigative team to interview witnesses and collect and preserve any evidence for future prosecutions, and recommend venues for pursuing justice. They urged using as a model the probe into the assassination of former Pakistani Prime Minister Benazir Bhutto, which was undertaken by the UN at the request of Islamabad.

“If the UN is truly mobilised to fight impunity for crimes against journalists, then at the very least they must be fully engaged in one of the most shocking and extreme cases in recent years by undertaking this investigation," Christophe Deloire, secretary-general of Reporters Without Borders, said in the statement

People hold signs during a protest at the Embassy of Saudi Arabia about the disappearance of Saudi journalist Jamal Khashoggi, in Washington on 10 October (AP)

The apparent killing of Mr Khashoggi has rattled dissident journalists and activists worldwide, and sparked fears of a wave of impunity. The Soufan Centre, an intelligence consultancy, warned of a wave of state-sponsored killings of political enemies that could increase if there are no consequences for Saudi Arabia.