Saudi opposition journalist Jamal Khashoggi is believed to have been killed within the confines of the consulate of Saudi Arabia in Istanbul.

Turkish President Recep Tayyip Erdogan called on Saudi Arabia to provide evidence that the missing journalist Jamal Khashoggi, who has been missing since last week, left the building of the Saudi consulate.

"We have to get an outcome from this investigation as soon as possible. The consulate officials cannot save themselves by simply saying 'he has left,'" Erdogan told a news conference in Budapest, where he is on an official visit, according to a Reuters report.

Speaking to journalists, Erdogan said he was personally monitoring the investigation, adding that so far the Turkish side has no documents or evidence regarding the case.

Over the last weekend, Turkey claimed that authorities believe that the journalist, who worked as an opinion columnist for the Washington Post, was killed inside the consulate building.

The Saudi consulate has denied that Khashoggi was killed at the mission and said that the accusations were false. The consulate also denied that Khashoggi had been abducted.

On Monday, Turkey officially asked for a permission to search the premises of the consulate in Istanbul. Earlier last week, Saudi Crown Prince Mohammed bin Salman said that he was ready to grant permission, saying Saudi Arabia "has nothing to hide."

Khashoggi was an opposition journalist in Saudi Arabia who criticized the royal family, including King Salman and Mohammad bin Salman, over freedom of politics, human rights and the ongoing war in Yemen.

Khashoggi, who left Saudi Arabia last year fearing arrest, came to the Saudi consulate in Istanbul to fetch documents related to his upcoming marriage, according to reports. According to the missing journalist‘s fiancée, 11 hours after entering the building Saudi guards told her that he had previously left. The Saudis deny all allegations and say Khashoggi left the building that day.

The disappearance of the journalist has caused concern to US President Trump.

"I am concerned about it. I don't like hearing about it. And hopefully that will sort itself out. Right now nobody knows anything about it, but there are some pretty bad stories going around. I do not like it," he told reporters at the White House, according to Reuters.