US President Donald Trump increased pressure on Saudi Arabia on Wednesday to provide information on what happened to missing Saudi journalist Jamal Khashoggi and said he wanted to get the bottom of what he called "a very serious situation."

In a fresh clue, a pro-government Turkish daily on Wednesday published preliminary evidence from investigators it said identified a 15-member Saudi intelligence team involved in Khashoggi's unexplained disappearance on October 2.

Pressure appeared to be building worldwide on close US ally Saudi Arabia on the whereabouts of Khashoggi, a prominent critic of Saudi policies, who was last seen entering the Saudi consulate in Istanbul to get documents for his planned marriage.

His Turkish fiancée, Hatice Cengiz, who was waiting outside, said he never re-appeared.

Turkish sources said they believe Khashoggi was killed inside the building and removed, allegations that Riyadh dismisses as baseless.

Trump, in comments in the Oval Office, told reporters he had raised Khashoggi's case with Saudi Arabia "at the highest level" and more than once in recent days.

"We're demanding everything," Trump said when asked if he was demanding information from the Saudis. "We want to see what's going on. It’s a very serious situation for us and for this White House. We want to get to the bottom of it."

He said he and his wife, Melania, expect to invite Cengiz to the White House soon.

"People saw him go in and didn't see him come out. We're going to take a very serious look at it. It's a terrible thing," Trump said. "This is a bad situation. We cannot let this happen, to reporters, to anybody.

We can't let this happen."

And in another sign the White House was stepping up pressure, White House national security adviser John Bolton and White House senior adviser Jared Kushner, Trump's son-in-law, spoke to Saudi Crown Prince Mohamed bin Salman on Tuesday.

The White House also said US Secretary of State Mike Pompeo followed up with his own call to the Crown Prince, who had forged close ties to the administration, especially Kushner.

"In both calls they asked for more details and for the Saudi government to be transparent in the investigation process. We will continue to monitor this situation and provide updates as available," White House spokeswoman Sarah Sanders said in a statement.

Khashoggi's fiancée Cengiz was surprised when an American friend told her of Trump's comment about inviting her to the White House, the friend told Reuters."I think she's in shock, now even more," her friend said.

In an essay in the Washington Post on Tuesday, Cengiz implored the president and Melania Trump to "help shed light on Jamal's disappearance."

Trump made Saudi Arabia the first stop on his first foreign trip as president in May 2017, but in recent weeks has appeared to sour a bit on Riyadh, complaining directly to King Salman about the cost of American support for the Saudi military and for Opec oil price increases.

One of the 15 Saudis identified by name and photo in the report by Turkey's Sabah newspaper is a forensic expert who has worked at the Saudi Interior Ministry for 20 years, according to a LinkedIn profile and Saudi media, and serves on the board of the Saudi Society of Forensic Medicine.

Other names and photos of the 15, who Sabah said travelled on diplomatic passports, match officers in the Saudi Army and Air Force, as identified by previous Saudi media reports and in one case a Facebook profile.

The Saudi consulate referred Reuters to authorities in Riyadh who have not responded to questions about the 15 Saudis, who arrived in the city hours before Khashoggi disappeared on October 2.

A Turkish security source had previously told Reuters that the 15 Saudis, including some officials, left Turkey later the same day.

The Washington Post reported on Tuesday that US intelligence had intercepted the communications of Saudi officials discussing a plan to capture Khashoggi, citing an unidentified person familiar with the information.

Yesterday, State Department spokesman Robert Palladino said the United States "had no advance knowledge" of such a plan.

Khashoggi left Saudi Arabia last year saying he feared retribution for his criticism of Riyadh over the Yemen war and its crackdown on dissent, and since then wrote columns for the Washington Post.

Britain's foreign minister called for urgent answers and the chairman of the US Senate Foreign Relations Committee Bob Corker, who has seen classified intelligence on the case, said information pointed to Khashoggi being killed.

"They've got some explaining to do," he said of Saudi Arabia.

Corker said he doubted accounts from Saudi officials that security cameras at the consulate did not record any images.

"To me it feels very much some nefarious activity has occurred by them. But I don't want to rush to judgment," Corker said on Tuesday.

US Vice President Mike Pence said on Wednesday the United States was ready to help in any way in the investigation.

Yesterday's Turkish media reports were the most detailed accounts yet of what they say is the Turkish government's investigation into Khashoggi's disappearance.

Sabah newspaper published the names and years of birth of the 15 Saudis it said arrived at Istanbul's Ataturk airport.

Twelve of them arrived early on Tuesday, based on photos captured at passport control which it published.

The 15 departed at four different times, Sabah reported.

It did not say how it obtained the pictures and data.

The Sabah report said that the Saudis stayed at the Wyndham and Movenpick hotels in Istanbul, in the same neighbourhood as the Saudi consulate.

Both hotels declined to comment.

Turkish NTV also broadcast security camera footage on Wednesday that it said showed the men arriving at the airport and checking into a hotel, as well as videos of what it said was a van arriving at the consul general's residence two hours after Khashoggi entered the consulate, about 250m (820ft) away.

Aksam newspaper also published blurred photos of a jet arriving in Istanbul before dawn on October 2, carrying what it said were nine members of the Saudi team, as well as images of some of them checking in at a hotel.

Aksam said they left their hotel later in the morning.

Shortly after midday it said several cars entered the consulate building.

A picture of Khashoggi entering the consulate shows the time-stamp 13:14.

About two hours later Aksam showed two vehicles, one of them a black Mercedes Vito van with tinted windows and diplomatic plates, leaving the consulate and entering the car park of the residence shortly afterwards.

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