Turkey will try the 20 suspects in the 2018 murder of journalist Jamal Khashoggi -- including two former aides to Saudi Crown Prince Mohammed bin Salman -- in absentia on Friday, his fiancee told AFP on Tuesday.

Khashoggi, 59, a Riyadh critic commentator who wrote for The Washington Post, was killed after he entered the Saudi consulate in Istanbul on October 2, 2018, to obtain paperwork for his wedding to Turkish fiancee Hatice Cengiz.

Turkish prosecutors accuse Saudi Arabia's deputy intelligence chief Ahmed al-Assiri and the royal court's media czar Saud al-Qahtani of leading the operation and giving orders to a Saudi hit team. 

Eighteen other suspects -- including intelligence operative Maher Mutreb who frequently travelled with the crown prince on foreign tours -- were also charged with "deliberately and monstrously killing, causing torment".

They face life in jail if convicted.

The trial in absentia will open at Istanbul's main court Caglayan on Friday at 10:00 am local time (0700 GMT), Khashoggi's fiance Cengiz told AFP late on Tuesday. "I will also be there," she said. 

Agnes Callamard, the UN special rapporteur on extrajudicial, summary or arbitrary executions, is also expected to attend the trial. 

There was no immediate official confirmation of the trial in absentia.

Turkish prosecutors had already issued arrest warrants for the Saudi suspects, who are not in Turkey.

Khashoggi's sons announced in May that they "forgive" the killers of their father. 

Khashoggi -- who was close to the Saudi royals but became a critic -- was killed and dismembered at the kingdom's consulate, in a case that tarnished the reputation of Crown Prince Mohammed.

His remains have never been found. 

Riyadh has described the murder as a "rogue" operation, but both the CIA and Callamard have directly linked Crown Prince Mohammed to the killing -- a charge the kingdom vehemently denies.