Saudi journalist Jamal Khashoggi was immediately strangled to death upon entering the Saudi consulate in Istanbul on October 2 before his body was dismembered and disposed an official Turkish investigation has found.

A statement released by Istanbul prosecutor Irfan Fidan’s office on Thursday morning (AEST) was the first public confirmation of Mr Khashoggi’s murder.

It acknowledged that despite Turkey’s “good-willed efforts” to uncover the truth, Mr Fidan struggled to reach any “concrete” conclusions about the death of Mr Khashoggi following a three-day long discussion with Saudi prosecutor Saud al-Mojeb and other Turkish officials.

Turkey has intensified its demands for Saudi Arabia to extradite 18 detained Saudi suspects who are being charged of “intentional homicide with premeditation” over Mr Khashoggi’s murder.

It is also seeking to gain information concerning the whereabouts of Mr Khashoggi’s remains and a disclosure revealing who ordered the journalist’s slaying.

Turkish President Recep Tayyip Erdogan has also urged Riyadh to divulge the identity of an alleged local collaborator believed to have assisted in the disposal of Mr Khashoggi’s body.

Khashoggi, a 59-year-old columnist for The Washington Post, vanished after entering the consulate in Istanbul, while his fiancee waited for him outside.

A critic of the Saudi royal family, he had been living in exile in the United States.

Turkey alleges a Saudi hit squad – including a member of Saudi Crown Prince Mohammed bin Salman’s entourage – travelled to Istanbul to kill the journalist and then tried to cover it up.

Under mounting pressure, Saudi Arabia changed its narrative about Khashoggi’s killing several times, eventually admitting that Khashoggi died inside the consulate and only recently acknowledging that Turkish evidence shows his killing was premeditated.

Saudi Arabia has not commented on its prosecutor’s visit.

-with AAP