Slain journalist Jamal Khashoggi was butchered in a "premeditated" attack, prosecutors say, in an extraordinary reversal from Saudi authorities about the fate of the writer.

Khashoggi, 59, hasn't been seen since he went into the Saudi consulate in Istanbul on October 2 to arrange paperwork for his upcoming wedding.

Turkey believes he was cut to pieces after being tortured and his remains hidden in the garden of the Saudi Consul General.

Saudi Arabia had called the murder a "huge and grave mistake", in which the Washington Post columnist was accidentally strangled after he was placed in a chokehold during an interrogation.

Now, a statement from Saudi Arabia's public prosecutor sent from the Saudi Foreign Ministry said a joint Saudi-Turkish investigation in Istanbul "indicates that the suspects in the incident had committed their act with a premeditated intention," reported the Post.

Saudi authorities maintained Crown Prince Mohammed bin Salman knew nothing about the murder, while Turkey claims it was a carefully planned assassination masterminded by the government.

In their earlier claim, the Saudi government — after denying any knowledge of what had happened to Khashoggi — said 18 people had been arrested after Khashoggi got into a fist fight.

US President Donald Trump initially said the claims were believable — but then distanced himself and labelled it "the worst cover-up ever".

The foreign ministry statement didn't say why prosecutors now believed the killing was planned, but obtained the information with the co-operation of Turkish officials. They would continue their investigation using the new details, the statement said.

Saudi Arabia's crown prince yesterday broke his silence on the sickening killing, saying the murder was a "heinous crime that cannot be justified".

Speaking at the Future Investment Initiative conference, nicknamed "Davos in the desert", the prince said all culprits would be punished, and vowed Saudi Arabia and Turkey would work together to find the killers.

Prince Mohammed said his country's national security agencies would be overhauled and "justice would prevail".

He told the audience: "The incident that happened is very painful, for all Saudis ... The incident is not justifiable.

"They will not be able to divide us as long as there is a king called King Salman bin Abdul Aziz and a crown prince named Mohammed bin Salman, and a president in Turkey named Erdoğan."

Turkey's President Recep Tayyip Erdogan pronounced the death a "savage murder", with Khashoggi killed and dismembered inside the consulate by Saudi agents on October 2. But Saudi authorities refused to allow Turkish police to search a well in the garden of the consulate in Istanbul, according to Turkey's state-run Andalou agency.

Turkish sources alleged that body parts and a disfigured face had been found in the garden, Sky News reported, but the discovery has not been confirmed.

Turkish authorities, including the president, said this week that Saudi agents had scouted Istanbul's Belgrad Forest before Khashoggi's death inside Saudi Arabia's Istanbul consulate on October 2.

Mr Erdogan says that when diplomats in the consulate learnt Khashoggi would be coming in for a document, one flew back to Riyadh, where "a road map started to be established."

A team of 15 Saudi agents then flew to Istanbul on October 2 with suitcases and a "bone saw", checking into hotels, but flying out the same day.

Mr Erdogan also confirmed that a body double of Khashoggi was used as a decoy after he was killed, after CCTV footage emerged on Monday of a man wearing the Washington Post columnist's dress shirt, suit jacket and pants.

Saudi officials admitted they brought in a body double to pose as Khashoggi, but claimed it was part of a plan to kidnap rather than to kill him.

Turkish officials also reportedly have a recording of the alleged "ferocious" torture and killing, but Mr Erdogan has held back any further evidence so far. He has also refrained from naming the crown prince directly, although he referred to King Salman's "sincerity".

Initially it was reported Khashoggi's Apple watch may have recorded his brutal murder — but now it seems likely Turkish spies were covertly recording what was happening in the consulate, either using a listening device hidden inside or by aiming a powerful microphone at the building from outside.

The crown prince and King Salman earlier invited Khashoggi's son Salah and brother Sahel to the Yamama Palace in Riyadh to offer condolences. The journalist's son, who is banned from leaving Saudi Arabia, was pictured staring coldly at the crown prince as they shook hands.