Washington (CNN) President Donald Trump said Monday that he was "concerned" about reports on the disappearance of journalist Jamal Khashoggi, a Washington Post writer and Saudi royal court insider-turned-critic.

"I am concerned about that," Trump said. "I don't like hearing about it and hopefully that will sort itself out. Right now, nobody knows anything about it."

"There's some pretty bad stories about it. I do not like it," he added.

Secretary of State Mike Pompeo echoed his comments in a statement late Wednesday, saying there were "conflicting reports," but that the US was "concerned" by Khashoggi's disappearance.

"We call on the government of Saudi Arabia to support a thorough investigation of Mr. Khashoggi's disappearance and to be transparent about the results of that investigation," Pompeo said.

'Absolutely false'

Trump's comments coincided with a vehement denial from Saudi Arabia's ambassador to the US, Prince Khalid bin Salman bin Abdulaziz, who said claims that Khashoggi had been killed or detained by Saudi authorities were "absolutely false, and baseless," according to a statement obtained by CNN.

"There are many facts regarding his whereabouts that will hopefully be revealed through the ongoing investigation. Despite that, we have seen over the last few days various malicious leaks and grim rumors flying around about Jamal's whereabouts and fate," the statement says.

"I assure you that the reports that suggest that Jamal Khashoggi went missing in the Consulate in Istanbul or that the Kingdom's authorities have detained him or killed him are absolutely false, and baseless," it notes.

Vice President Mike Pence also weighed in on Khashoggi's disappearance, saying on Twitter, "Deeply troubled to hear reports about Saudi Arabian journalist Jamal Khashoggi. If true, this is a tragic day. Violence against journalists across the globe is a threat to freedom of the press & human rights. The free world deserves answers."

It was the first time Trump has weighed in on the situation involving Khashoggi, who was last seen entering the Saudi consulate in Istanbul last Tuesday.

CNN on Tuesday obtained an image from a security camera of Khashoggi entering the building on October 2 at 1:14 p.m. local time.

CCTV image of Khashoggi entering the consulate on October 2.

Earlier on Monday, Turkish President Recep Tayyip Erdogan said Saudi Arabia should prove that Khashoggi has, in fact, left the Saudi consulate in Istanbul.

"He entered the general consulate himself, and if he has entered by himself and if he did not exit it, of course this should be proven by the general consulate," Erdogan said at a news conference in Budapest.

Erdogan said the Saudi consulate should have surveillance cameras and should be able to show the video of Khashoggi leaving the building. He mentioned that there are no documents or other evidence that show the journalist departing.

Turkish officials told The Washington Post and Reuters on Saturday that Khashoggi was killed inside the consulate. The officials have so far provided no evidence or details on how they arrived at this conclusion.

"I am following it up as the President of the Turkish Republic," Erdogan told reporters in Ankara on Sunday, while confirming that he had known Khashoggi for some time and considered him a friend.

The Justice Ministry and the chief prosecutor in Istanbul "started an investigation and efforts are continuing," Erdogan said. Airport entrances and exits are being investigated.

"At the moment there are certain people who arrived from Saudi Arabia. And our chief investigator is investigating everything in this matter."

No surveillance footage

Khashoggi, who left Saudia Arabia in 2017, entered the consulate last Tuesday to obtain documents for his upcoming marriage while his Turkish fiancée waited outside. She says she never saw him re-emerge.

A Saudi official said Khashoggi left the consulate shortly after he visited. The Saudis did not, however, release any surveillance footage or other evidence.

CNN reported Sunday that the US government is quietly working Khashoggi's case across several agencies and at senior levels of the administration.

Two senior administration officials said the administration has no verifiable information to confirm the Turkish government claims that Khashoggi was killed but that the US is seeking answers about his whereabouts -- and talking to senior levels of Saudi government.

In his statement to CNN, Saudi Arabia's ambassador to the US said Saudi investigators are working with Turkish authorities to determine what happened to Khashoggi.

"Jamal is a Saudi citizen whose safety and security is a top priority for the Kingdom, just as is the case with any other citizen. We will not spare any effort to locate him, just as we would if it were any other Saudi citizen," it said.

The incident has put into sharp focus Saudi Arabia's young Crown Prince Mohammed bin Salman's perceived crackdown on dissidents, his kingdom's delicate relationship with Turkey and Khashoggi 's influence within the royal court.