SAUDI Arabia's public prosecutor said he was seeking the death penalty for five out of 11 suspects charged in the murder of prominent journalist Jamal Khashoggi.

Khashoggi, a critic of Saudi policy, was killed in the country's Istanbul consulate on October by a lethal injection dose after a struggle and his body was dismembered and taken out of the building, said the prosecutor.

The 59-year-old Washington Post journalist was allegedly killed by a 15-man Saudi team.

Deputy public prosecutor and spokesman Shaalan al-Shaalan said Khashoggi was murdered after "negotiations" for his return to the kingdom failed.

The whereabouts of Khashoggi's body remain unknown, he said.

Khashoggi's killers had set in motion plans for the killing on September 29, three days before he was killed.

The spokesman said the highest-level official behind the killing is Saudi former deputy intelligence chief Ahmad al-Assiri, who has been fired for ordering Khashoggi's forced return.

Al-Shaalan added that a travel ban has been imposed on a top aide to the crown prince, Saud al-Qahtani, while investigations continue over his role.

Qahtani had met the team ordered to repatriate Kashoggi ahead of their journey to Istanbul to brief them on the journalist's activities, said al-Shaalan.

The aide has already been fired from the royal court.

A total of 21 people are now in custody, with 11 indicted and referred to trial, said the prosecutor.

Al-Shaalan also said the country's powerful Crown Prince Mohammed bin Salman "did not have any knowledge about it".

The circumstances surrounding Khashoggi's death sparked controversy, with global leaders blasting the Saudi government's sometimes contradictory version of events.

In the aftermath of his death, later a man strolled out of the diplomatic post apparently wearing the columnist's clothes as part of a macabre deception to sow confusion over the writer's fate.

Surveillance video leaked to CNN showed the imposter wearing Khashoggi's shirt, suit jacket and trousers, although he wore a different pair of shoes.

It cited a Turkish official as describing the man as a "body double" and a member of the Saudi team sent to Istanbul to target the writer.

It has been widely reported Khashoggi entered the Saudi consulate in Istanbul shortly after 1pm on October 2.

He wanted to obtain a document certifying he had divorced his ex-wife, so that he could marry his Turkish fiancée Cengiz.

She waited outside - and never saw him again.

Turkish Foreign Minister Mevlut Cavusoglu is not satisfied by some of the remarks made by Saudi Arabia's public prosecutor.

He repeated Turkey's demand that the 15-man team involved in Khashoggi's killing should be tried in Turkey.

Cengiz later revealed that he had told her to call an adviser to Turkish President Recep Tayyip Erdogan if he did not return.

The Saudi government remained silent for nineteen days, before initially claiming he died during a fight inside the Saudi consulate.

Their claims followed reports from Turkish investigators and intelligence agencies that it was a premeditated strike.

It has been reported that Khashoggi was deliberately tortured and killed in a disturbing seven-minute execution.