ISTANBUL - Turkey on Friday widened the investigation into the disappearance of journalist Jamal Khashoggi after his visit to the Saudi consulate, searching a forest in the city and interviewing the mission's staff.

Ankara denied giving any audio recording to US officials from the investigation about Khashoggi, a former royal insider who moved to the United States after becoming a critic of the current House of Saud leadership.

US President Donald Trump acknowledged that Khashoggi was likely dead, even as his fate remained unclear 17 days after he vanished. "It certainly looks that way to me. It's very sad," Trump told journalists when asked if he believed that Khashoggi, who disappeared more than two weeks ago, is no longer alive.

Asked about the potential US response to Saudi Arabia, which is accused of murdering the Washington Post columnist and critic of the Saudi regime, Trump said: "It will have to be very severe. It's bad, bad stuff."

Pro-government Turkish media have repeatedly claimed that Khashoggi was tortured and decapitated by a Saudi hit squad inside the consulate, although Turkey has yet to divulge details about the investigation. But the controversy has already put the kingdom - for decades a key Western ally and bulwark against Iran in the Middle East - under unprecedented pressure amid reports it is scrambling to provide an explanation to take the heat off its rulers.

It is also a major crisis for Crown Prince Mohammed bin Salman, a favourite of the Trump administration who has portrayed himself as a modernising Arab reformer, but whose image and even position at home could now be gravely undermined.

Close ally the United Arab Emirates warned on Friday the controversy should not be exploited to "destabilise" Saudi Arabia. Fifteen staff, all Turkish nationals, were testifying Friday at the chief prosecutor's office, state-run news agency Anadolu said. It has been reported Turkish employees were given the day off on October 2, the day Khashoggi disappeared.

Among those giving statements inside Istanbul's main courthouse were the consulate driver, technicians, accountants and receptionists. Istanbul's Belgrad forest became a target of the investigation after police focused on the vehicles which had left the consulate on the day Khashoggi disappeared, NTV television reported. At least one vehicle is suspected to have gone to the forest.

The forest, a vast area and sufficiently remote for even locals to regularly get lost there, is nearly 15 kilometres (over nine miles) from the consulate.

Investigators already conducted two searches of the consulate and a nine-hour search of the consul's residence this week.

Pro-government daily Sabah on Friday published new CCTV images of some of the Saudi team arriving in Istanbul and reported that two of the men landed in the city on October 1.

Previously, local media said the 15 men arrived in Turkey on October 2 on two private planes.

Foreign Minister Mevlut Cavusoglu did not reveal probe details but promised to share information in due course "in a transparent manner".

"It is out of the question for us to share this or that information with any country," he said.

The key potential piece of evidence in the investigation is an alleged audio tape whose existence has been reported by pro-government media. They say it proves Khashoggi was tortured and then killed.

ABC News on Thursday quoted an unnamed Turkish official saying US Secretary of State Mike Pompeo heard the audio tape and was shown a transcript of the recording during his visit to Ankara.

But Pompeo said he had neither "seen" nor "heard" a tape and had not read a transcript while in Ankara where he met with President Recep Tayyip Erdogan and Cavusoglu.

Cavusoglu on Friday also denied the claims and said it was "out of the question for Turkey to give any kind of audio tape to Pompeo or any other US official".

Previously US media said Saudis were preparing a report that Khashoggi's death resulted from a botched interrogation.

As Washington seeks to avoid a long-term rupture with its ally Riyadh, Pompeo told Trump the Saudis should be given "a few more days to complete" an official probe.

The furore has also blown a huge hole in next week's Future Investment Initiative conference in Riyadh. It was meant to showcase Prince Mohammed's plans for reform but has now been hit by a stream of big name cancellations including US Treasury Secretary Steven Mnuchin.

But the UAE state minister for foreign affairs, Anwar Gargash, tweeted that Abu Dhabi was "firmly" opposed to "politicisation (of the case) and efforts to destabilise SaudiArabia."