Turkish President Recep Tayyip Erdogan has pledged to describe "in detail" what happened to dissident Saudi journalist Jamal Khashoggi during a speech before members of his Justice and Development Party (AKP) on Tuesday.

The information must come out "because we are seeking justice," Erdogan said in Istanbul on Sunday.

The death of Khashoggi inside the Saudi consulate in Istanbul on October 2 was described as "murder" for the first time by Saudi Foreign Minister Adel al-Jubeir on Sunday.

Riyadh had initially denied that Saudi officials had any knowledge of what had happened to the 59-year-old dissident journalist, saying he had left the consulate after picking up papers for his forthcoming marriage.

The kingdom admitted on Saturday - 18 days later - that Khashoggi indeed died in the consulate but blamed the death on a fistfight and said more than a dozen people were being questioned.

Turkish media have reported that Khashoggi was murdered and dismembered based on recordings from the consulate. They say he was killed by a 15-member squad from Saudi Arabia.

Khashoggi's death was a mistake and those responsible will be held to account, Saudi Arabia's al-Jubeir told Fox News on Sunday.

"There obviously was a tremendous mistake made and what compounded the mistake was the attempt to try to cover up. That is unacceptable in any government," al-Jubeir said.

"We are determined to find out all the facts and we are determined to punish those who are responsible for this murder," he said.