Five people have been sentenced to death over the killing of Saudi journalist Jamal Khashoggi, Saudi Arabia's public prosecutor says.

Three more have been given jail terms totalling 24 years after the murder inside the Saudi consulate in Istanbul in October last year.

Saudi Arabia has carried out the trials of the accused in near total secrecy, after Saudi's crown prince drew international condemnation for the killing because several Saudi agents who worked directly for him were involved.

The crown prince's former top adviser, Saud al-Qahtani, had no proven involvement in the killing, Saudi's state-run Al-Ekhbariya TV reported.

Mr Al-Qahtani has been sanctioned by the US for his alleged role in the operation.

The court also found the Saudi consul-general in Istanbul, Mohammed al-Otaibi, not guilty.

He and nine others, who were not named, were released from prison after the verdicts were announced, according to state TV.

The trial concluded after nine sessions that there was no previous intent to murder.

A handful of Saudi diplomats attended the trial but it was closed to the press, the wider public and members of Mr Khashoggi's family.

Officials seen outside the Saudi Arabian consulate in Istanbul, Turkey in October 2018 | Image: DHA/ABACA/ABACA/PA Images

The journalist, a Saudi dissident and US resident who worked for the Washington Post, walked into the Saudi consulate in Istanbul on October 2nd 2018 to obtain documents he needed to get married.

He was drugged and suffocated before his body was dismembered soon after entering the building.

Recordings made before and during the killing, obtained by Turkish intelligence authorities, were published in September and revealed Mr Khashoggi's final words were a plea to his killers not to cover his mouth because he suffered from asthma and could suffocate.

According to the report, in the moments before he was killed, Mr Khashoggi had pleaded: "Don't cover my mouth. I have asthma, don't do it. You'll suffocate me."

#STATEMENT by the Public Prosecutor regarding the murder of citizen Jamal bin Ahmed bin Hamza Khashoggi (may his soul Rest In Peace). pic.twitter.com/745x7OABkx

— Foreign Ministry 🇸🇦 (@KSAmofaEN) December 23, 2019

Included in the transcript are discussions of how Mr Khashoggi would be killed and his body sawed into pieces.

Minutes before the journalist arrived at the consulate, Dr Salah Muhammed Al-Tubaigy, the head of forensic evidence at the Saudi General Security Department, was recorded saying: "Actually, I've always worked on cadavers. I know how to cut very well. I have never worked on a warm body though, but I'll also manage that easily.

"I normally put on my earphones and listen to music when I cut cadavers. In the meantime, I sip on my coffee and smoke.

"After I dismember it, you will wrap the parts into plastic bags, put them in suitcases and take them out."

A report on his murder by the United Nations, released in June, blamed the Saudi state for his death, saying the crown prince's possible role should be examined.

Mr Khashoggi's remains have never been found.