In a speech to parliament about a case that has sparked outrage around the globe, Erdogan did not mention Crown Prince Mohammed bin Salman, who some US politicians suspect ordered the killing.

But he said Turkey would not complete its investigation into Khashoggi's death until all questions were answered.

"Intelligence and security institutions have evidence showing the murder was planned. Pinning such a case on some security and intelligence members will not satisfy us or the international community," he said.

"From the person who gave the order, to the person who carried it out, they must all be brought to account."

Erdogan said the whereabouts of Khashoggi's body were still unknown and he demanded Saudi Arabia reveal the identity of a "local cooperator" who purportedly took the body.

Erdogan's speech coincided with the opening of an investment conference which Western political figures, leading international bankers and company executives have boycotted because of the furore around Khashoggi's death.

The Washington Post columnist, a sharp critic of the crown prince, the kingdom's de facto ruler, disappeared three weeks ago after entering the Saudi consulate in Istanbul to obtain documents for his upcoming marriage.

Turkish officials suspect Khashoggi was killed and dismembered inside the consulate by Saudi agents.

Turkish sources say authorities have an audio recording purportedly documenting the killing of the 59-year-old. Erdogan made no reference to any audio recording in his speech.

Riyadh initially denied knowledge of his fate before saying he was killed in a fight in the consulate. That version of events was greeted sceptically by several Western governments, straining relations with the world's biggest oil exporter.

Erdogan said three operatives arrived in Istanbul the day before his killing on an apparent reconnaissance mission. The next day 15 people came to the consulate. They included security, intelligence and forensic experts, and consulate personnel were given the day off.

"Why did these 15 people meet in Istanbul on the day of the murder? We are seeking answers to this. Who are these people receiving orders from?" Erdogan said.

Representatives for the White House and the US Department of State did not immediately respond to a request for comment on Erdogan's remarks

Following the global outrage prompted by the journalist's disappearance, US President Donald Trump's comments have varied from playing down Riyadh's role to warning of possible economic sanctions.

Trump has also repeatedly highlighted the kingdom's importance as a US ally and said Prince Mohammed was a strong and passionate leader.

Australian Associated Press