Five people have been sentenced to death over the murder of journalist Jamal Khashoggi, but two top figures investigated over the killing have been exonerated, Saudi Arabia's public prosecutor said.

Asked if he ordered the murder of Khashoggi, who had criticized him in columns for The Washington Post, Prince Mohammed bin Salman replied: "Absolutely not." The slaying was "a mistake," he said.

Prince Mohammed, the kingdom's de facto ruler, has come under huge international pressure after the US-based writer was killed and dismembered in the Saudi consulate in Istanbul.

Pompeo's trip comes despite growing controversy inside the United States, over the Trump administration's cosy relationship with Mohammed bin Salman after US-based dissident writer, Jamal Khashoggi was killed and dismembered in the Saudi consulate in Istanbul last year.

The expert has said that United Nations Secretary-General Antonio Guterres should launch an international criminal investigation into the killing of Saudi journalist Jamal Khashoggi.

'Any party exploiting the case politically should stop doing so, and present evidence to the (Saudi) court, which will contribute to achieving justice,' Saudi Crown Prince Mohammed bin Salman said, without directly naming Turkey.

The body of slain Saudi journalist Jamal Khashoggi was likely burned in a large oven at the Saudi consulate general's residence in Istanbul.

The dissident journalist was dismembered after his murder October 2 in the Saudi consulate in Istanbul but his remains have yet to be found.

US media report quotes intelligence sources who intercepted a conversation between Mohammed bin Salman and an aide in 2017.

Secretary of State Mike Pompeo will continue to press the Saudis on Khashoggi's murder when he visits Riyadh next week as part of an eight-country tour of the Middle East.