US President Donald Trump threatened Riyadh on Saturday with "severe punishment" if journalist Jamal Khashoggi was killed inside its Istanbul consulate, as Saudi officials were to hold talks in Turkey about his fate.

With the mystery over Jamal Khashoggi unresolved 11 days after he walked into the consulate and failed to reappear, a pro-government Turkish daily said the Saudi national had recorded his own interrogation inside the mission on an Apple Watch.

Turkish officials have said they believe Khashoggi was killed inside the consulate and lurid claims have been leaked to media that he was tortured and even dismembered.

Saudi insists Khashoggi, a Washington Post contributor whose writings have been critical of powerful Crown Prince Mohammed bin Salman, left the building safely but has yet to offer visual evidence of this.

The outcry surrounding his disappearance threatens to not just harm brittle Turkey-Saudi relations but also alarm the kingdom's supporters in the West and tarnish the reform drive spearheaded by the crown prince.

"We're going to get to the bottom of it and there will be severe punishment," Trump told CBS's "60 Minutes" program, according to an extract of an interview that was released on Saturday.

"As of this moment, they (Saudi) deny it and they deny it vehemently. Could it be them? Yes," Trump said in the interview, which was conducted on Thursday.

But he again voiced his reluctance to limit US arms sales to the kingdom, which analysts see as one of Washington's key potential levers.

Trump, who has become notorious for his attacks on American journalists, added the matter was especially important "because this man was a reporter".

The Saudi delegation was in Turkey and due to have talks this weekend in Ankara and take part in a working group on the disappearance, official Turkish media said. The NTV channel said the 11 person delegation had on Friday inspected the consulate in Istanbul.

Riyadh has warmly welcomed the creation of the working group but Interior Minister Prince Abdel Aziz bin Saud bin Nayef slammed claims that the kingdom ordered Khashoggi to be killed inside the consulate.

He described the allegations as "baseless allegations and lies".

Ankara has so far trodden carefully in the controversy, with the most sensational allegations splashed in the pro-government press but President Recep Tayyip Erdogan has so far stopped short of directly accusing Riyadh of wrongdoing.

Turkey and Saudi have an uneasy relationship, with disputes over the ousting of the Islamist government in Egypt and the blockade imposed on Ankara's ally Qatar.

But Erdogan has generally been wary of needling the oil-rich conservative kingdom and on Saturday again gave a long speech to supporters without mentioning the issue.

The spokesman of Erdogan's ruling party, Omer Celik, acknowledged Saturday that there were "extremely sensational claims" about Khashoggi's fate in the media and said there would be "severe consequences" for anyone found responsible if they were true.

"Far from the speculation, work is being carried out in the most sensitive way to find out what happened," he said in televised comments. 

The latest claims reported by the pro-government Sabah daily said that Khashoggi had been wearing an Apple Watch when he entered the consulate which was synced with an iPhone left outside with his fiancee Hatice Cengiz.

It said that the watch had recorded what happened inside the consulate and this was uploaded to his cloud, although Saudis sought to partially delete it.

"The moments of Khashoggi's questioning, torture and killing were recorded on the Apple watch," said Sabah.

Analysts say that Turkey is hoping to find support from its NATO ally the United States in the case, although Ankara-Washington have been in crisis over the detention for the last two years of a Protestant pastor.

But the pastor, Andrew Brunson, was freed on Friday and allowed to fly home by a Turkish court, in a move that could help normalise ties. 

Meanwhile, Prince Mohammed's big October conference -- the Future Investment Initiative dubbed by media as the "Davos in the Desert" after the annual conference in the Swiss resort -- has suffered a litany of cancellations over the controversy.

Key business figures like the chief executive of ride-hailing app Uber -- into which the Saudi's own investment fund injected money -- are no longer showing up while media groups like the New York Times, Financial Times and Bloomberg have pulled their sponsorship.

US Treasury Secretary Steven Mnuchin said Saturday that he still planned to attend, as did IMF chief Christine Lagarde.

She said she was "horrified" by the case but has to "conduct the business of the IMF in all corners in the world and with many governments".