Police searching the Saudi Consulate "found evidence" that Saudi writer Jamal Khashoggi was killed there, according to a high-level Turkish official.

Authorities appeared ready to search the nearby residence of Consul General Mohammed al-Ota after the diplomat left the country on Tuesday as the comment by an unnamed Turkish further intensified the pressure on Saudi Arabia to explain what happened to Khashoggi.

The high-level Turkish official told The Associated Press that police found "certain evidence" of Khashoggi's slaying at the consulate, but would not elaborate. The official spoke on condition of anonymity because the investigation was ongoing.

Khashoggi vanished on October 2 while visiting the consulate to pick up paperwork he needed to get married.

Jamal Khashoggi disappeared on OaSaudi Arabia are preparing to announce journalist Jamal Khashoggi, who disappeared after visiting the country's Istanbul consulate was killed in an 'interrogation gone wrong'.ct. 2, 2018, after visiting his country's consulate in Istanbul and may have been killed there (AP Photo/Hasan Jamali, File)

President Donald Trump said after a phone call with Crown Prince Mohammed bin Salman that he "totally denied any knowledge of what took place in their Turkish Consulate."

The crown prince "told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly," Trump said in a tweet.

This comes as US Secretary of State Mike Pompeo visited Saudi Arabia to talk to King Salman and the 33-year-old crown prince about the fate of the journalist who wrote critically about the Saudis for the Washington Post.

While it was all smiles and handshakes in Riyadh, one prominent Republican senator said he believed that the crown prince, widely known as MBS, had Khashoggi "murdered."

"This guy has got to go," said Senator Lindsey Graham, R-South Carolina, speaking on Fox television.

"Saudi Arabia, if you're listening, there are a lot of good people you can choose, but MBS has tainted your country and tainted himself."

Saudi officials have called Turkish allegations that a team of 15 Saudi agents killed Khashoggi "baseless," but US media reports suggested that the kingdom may acknowledge the writer was killed at the consulate, perhaps as part of a botched interrogation.

The close US ally is ruled entirely by the Al Saud monarchy, and all major decisions in the ultraconservative kingdom are made by the royal family.

Washington Post Publisher and CEO Fred Ryan said the Saudi government "owes the Khashoggi family and the world a full and honest explanation of everything that happened to him," noting that Tuesday marked two weeks since the disappearance of the 59-year-old journalist.

"The Saudi government can no longer remain silent, and it is essential that our own government and others push harder for the truth," Ryan added.

US Secretary of State Mike Pompeo meets with the Saudi Crown Prince Mohammed bin Salman in Riyadh, Saudi Arabia. (AAP)

Police planned a second search at the Saudi consul general's home, as well as some of the country's diplomatic vehicles, Turkey's Foreign Minister Mevlut Cavusoglu said.

Leaked surveillance video show diplomatic cars travelled to the consul general's home shortly after Khashoggi went into the consulate.

Consul General Mohammed al-Otaibi left Turkey Tuesday afternoon, state media reported, just as police began putting up barricades around his official residence.

Saudi Arabia did not immediately acknowledge he had left or offer a reason for his departure.

Earlier in the day, UN human rights chief Michelle Bachelet said the "inviolability or immunity" of people or premises granted under the 1963 Vienna Convention on Consular Relations "should be waived immediately."

That convention covers diplomatic immunity, as well as the idea that embassies and consulates sit on foreign soil in their host countries.

"Given there seems to be clear evidence that Mr Khashoggi entered the consulate and has never been seen since, the onus is on the Saudi authorities to reveal what happened to him," Bachelet said.

Turkey had wanted to search the consulate for days.

CCTV allegedly shows Saudi journalist Jamal Khashoggi (right) entering the Saudi consulate in Istanbul, Turkey, before he vanished. (AAP/SABAH NEWSPAPER)

Permission apparently came after a late Sunday night call between King Salman and Turkish President Recep Tayyip Erdogan.

Certain areas of the consulate were to remain off-limits, although officials would be able to inspect surveillance cameras, Turkish media reported.

Erdogan told journalists Tuesday that police sought traces of "toxic" materials and suggested parts of the consulate had been recently painted, without elaborating.

In Riyadh, Saudi Foreign Minister Adel al-Jubeir greeted Pompeo at the airport.

The former CIA chief didn't make any remarks to the media.

Soon after, Pompeo arrived at a royal palace, where he thanked King Salman "for accepting my visit on behalf of President Trump" before the two went into a closed-door meeting.

Pompeo then met a smiling Prince Mohammed, the heir apparent to the throne of the world's largest oil exporter.

"We are strong and old allies," the prince told Pompeo.

"We face our challenges together — the past, the day of, tomorrow."

According to one source, the interrogation operation may have been carried out with clearance and transparency. (AAP)

Pompeo was to have a dinner Tuesday night with Prince Mohammed and was expected to fly to Turkey on Wednesday.

Trump previously warned of "severe punishment" for the kingdom if it was found to be involved in Khashoggi's disappearance, which has spooked investors.

Trump's warning drew an angry response Sunday from Saudi Arabia and its state-linked media, including a suggestion that Riyadh could wield its oil production as a weapon.

The US president has been after King Salman and OPEC to boost production to drive down high oil prices, caused in part by the coming re-imposition of oil sanctions on Iran.

On Monday, however, Trump offered a different theory after speaking by telephone with King Salman.

"It sounded to me like maybe these could have been rogue killers," Trump said.

"I mean, who knows? We're going to try getting to the bottom of it very soon, but his was a flat denial."

The New York Times and the Washington Post have reported, citing anonymous sources, that Saudi officials may soon acknowledge Khashoggi's slaying at the consulate but blame it on a botched intelligence operation.

That could, like Trump's comments, seek to give the kingdom a way out of the global firestorm of criticism over Khashoggi's fate.

"The effort behind the scenes is focused on avoiding a diplomatic crisis between the two countries and has succeeded in finding a pathway to deescalate tensions," said Ayham Kamel, the head of the Eurasia Group's Mideast and North Africa division.