Turkish authorities believe that prominent Saudi journalist Jamal Khashoggi, who disappeared four days ago after entering Saudi Arabia's consulate in Istanbul, was killed inside the consulate, two Turkish sources said on Saturday.

"The initial assessment of the Turkish police is that Mr. Khashoggi has been killed at the consulate of Saudi Arabia in Istanbul. We believe that the murder was premeditated and the body was subsequently moved out of the consulate," one Turkish official told Reuters.

The sources did not say how they believed the killing was carried out. Saudi Arabia's consul-general told Reuters on Saturday his country was helping search for Khashoggi, and dismissed talk of his possible abduction.

Khashoggi, who has lived in self-imposed exile in Washington for the past year fearing retribution for his critical views on Saudi policies, entered the consulate on Tuesday to secure documentation for his forthcoming marriage, according to his fiancee, who waited outside. He has not been heard of since.

Since then, Turkish and Saudi officials have offered conflicting accounts of his disappearance, with Ankara saying there was no evidence that he had left the diplomatic mission and Riyadh saying he exited the premises the same day.

Earlier on Saturday Turkish officials said prosecutors had begun investigating Khashoggi's disappearance and a spokesman for President Tayyip Erdogan's AK Party said authorities would uncover his whereabouts.