US President Donald Trump said Wednesday he has talked to Saudi authorities "at the highest level" to demand answers over what happened to vanished journalist Jamal Khashoggi.

Trump told reporters at the White House that he has talked to the Saudi leadership "more than once" since Khashoggi, a US resident and Washington Post contributor, vanished on October 2 after entering the Saudi consulate in Istanbul.

"We're demanding everything. We want to see what's going on there," he said. "It's a very serious situation for us and this White House ... I think we'll get to the bottom of it."

"We cannot let this happen, to reporters, to anybody," Trump said.

These were the toughest comments coming out of the Trump administration since the mysterious disappearance of the journalist, who Turkish authorities suspect was abducted and murdered by the Saudis.

Turkish investigators are examining CCTV footage showing the moment he entered the kingdom's consulate in Istanbul to obtain documents ahead of his marriage to his Turkish fiance Hatice Cengiz.

Cengiz appealed to Trump in an opinion piece for the Washington Post on Tuesday, calling on him to "help shed light on Jamal's disappearance."

Trump said that he and First Lady Melania Trump were in contact with her and were looking to "bring her to the White House."

- Suspicious vehicle -

Turkish government sources said at the weekend that police believed Khashoggi was killed by a team specially sent to Istanbul and thought to consist of 15 Saudis.

Security camera footage released on Wednesday by Turkish television showed a man believed to be Khashoggi enter the consulate. Investigators are also zooming in on a vehicle seen entering and leaving the building after Khashoggi went inside.

But Riyadh insisted the 59-year-old journalist had left the building and that murder claims are "baseless".

Khashoggi, a former Saudi government adviser, had been living in the United States since last year fearing arrest.

He has been critical of some policies of the Saudi Crown Prince Mohammed bin Salman and Riyadh's intervention in the war in Yemen.

Turkish police were looking into two private aircraft that landed at Istanbul's Ataturk airport on October 2 at different times carrying the individuals of interest in the case.

A source told the Washington Post that US intelligence "intercepted communications of Saudi officials discussing a plan to capture him."

The same source said the Saudis hoped to "lure" Khashoggi to Saudi Arabia "and lay hands on him there".

- Possible kidnapping -

One of the first images from the CCTV footage shared by 24 TV broadcaster showed Khashoggi enter the consulate at 1:14 pm (1014 GMT).

Footage also showed some of the Saudis arriving in Istanbul after the first plane landed before 0030 GMT on October 2 and the men later checking into a hotel near the consulate. Aksam daily said some of the men went into the Saudi consulate before Khashoggi.

According to the images, a vehicle that went inside the consulate was then driven to the consul-general's residence nearby after 1200 GMT, two hours after Khashoggi had entered the mission.

Aksam newspaper's editor-in-chief Murat Kelkitlioglu speculated on 24 TV that it was "almost certain" that Khashoggi had been taken in the vehicle.

Local media on Tuesday reported on the possibility that Khashoggi was kidnapped and taken aboard one of the private planes.

Both planes later returned to Riyadh with one stopping in Dubai and the other in Egypt, pro-government Sabah daily said.

According to Hurriyet daily, nine Saudis who arrived in Istanbul on the same day that the journalist vanished, had bought luggage at the Grand Bazaar. However, a police search revealed that they did not take the luggage on their return.

Sabah daily on Wednesday published the names and images of what it called the "assassination team" including a man called Salah Muhammed Al-Tubaigy whose name it said matched that of a lieutenant-colonel in the Saudi forensic department.

Sabah added that no "body parts" appeared on scans of the belongings of seven passengers of relevance to the case at Istanbul airport.

Turkey has said Saudi authorities gave officials the greenlight to search the consulate but it has not yet taken place.

- 'I shouldn't go' -

In his last interview three days before his disappearance, Khashoggi said that he did not think he would return to Saudi Arabia.

"When I hear of the arrest of a friend who did nothing that (deserved being) arrested, it makes me feel I shouldn't go," he told the BBC.

Reporters Without Borders (RSF), which ranks the kingdom 169th out of 180 on its World Press Freedom Index, said in a statement that between 25 and 30 professional and non-professional journalists are currently detained in Saudi Arabia.

RSF said at least 15 Saudi journalists and bloggers have been arrested since September 2017.