ISTANBUL - Saudi Arabia on Tuesday agreed to let Turkish authorities search its Istanbul consulate as part of the investigation into the disappearance of journalist Jamal Khashoggi, a critic of the kingdom's Crown Prince Mohammed bin Salman.

There has been fierce speculation over what happened to Khashoggi, a Washington Post contributor who wrote for Arab and Western media, after he vanished last Tuesday following a visit to the consulate to obtain official documents.

While Riyadh claimed he had left the consulate after his visit, Turkish police said Khashoggi did not emerge from the building.

Government sources said police believe the journalist was killed inside the consulate, claims which Riyadh dismissed as "baseless".

"Saudi authorities said they were open to cooperation and that a search can be conducted at the consulate building," the Turkish foreign ministry spokesman Hami Aksoy said in a statement.

The search will take place as part of the official investigation, which was being conducted "in an intense manner", he said without giving any date.

Turkish police were looking into two private aircraft which landed at Istanbul's Ataturk airport on Tuesday at different times carrying 15 people of interest in the case, as well as the possibility that Khashoggi was kidnapped and taken aboard one of the planes, local media reported. Previously the crown prince told Bloomberg that Riyadh would be ready to welcome Turkish officials to search the premises.

Turan Kislakci, a friend of Khashoggi and chief of the Turkish-Arab Media Association, urged the Turkish authorities not to search the consulate.

"We ask the consulate and officials to share the footage of his exit. We don't want the search or inspection of the consulate. We don't have such a demand," he told AFP. Ankara sought permission Sunday to carry out a search after the foreign ministry summoned the Saudi ambassador for a second time, Turkish television reported.

Riyadh's envoy in Ankara was first called to the ministry on Wednesday.

Khashoggi, a former Saudi government adviser, had been living in self-imposed exile in the United States since last year fearing possible arrest. He has been critical of some policies of the crown prince and Riyadh's intervention in the war in Yemen.

Police also said a team of 15 Saudis were sent to Istanbul and were in the building at the same time as Khashoggi.

US President Donald Trump told reporters at the White House on Monday that he was "concerned", adding: "I don't like hearing about it. Hopefully that will sort itself out." "I am concerned. I don't like hearing about it. Hopefully that will sort itself out," Trump told reporters at the White House. "Right now, nobody knows anything about it. There are some pretty bad stories going around. I do not like it."

US Secretary of State Mike Pompeo on the same day called for "a thorough investigation of Mr Khashoggi's disappearance and to be transparent about the results" of the probe. "We have seen conflicting reports on the safety and whereabouts of prominent Saudi journalist and Washington Post contributor Jamal Khashoggi," Pompeo said. UN rights office spokeswoman, Ravina Shamdasani, on Tuesday echoed calls for "a prompt, impartial and independent investigation of the circumstances of Mr Kh­ashoggi's disappearance and to keep the findings public" in comments to journalists in Geneva. "We are 100 percent behind the American position. We expect a thorough investigation and total transparency from the Saudi authorities on what happened," European Union's foreign policy chief Federica Mogherini told reporters in Lisbon.

Meanwhile, in London, British Foreign Secretary Jeremy Hunt met the Saudi ambassador on Tuesday to "seek urgent answers", according to a post on Twitter.