A United Nations human rights investigator leading an international inquiry into the murder of Saudi journalist Jamal Khashoggi said on Saturday she requested access to Saudi Arabia's consulate in Istanbul, and to visit the kingdom.

Agnes Callamard, UN special rapporteur on executions, who begins a week-long mission to Turkey at the government's invitation on Monday, said she had not yet received a reply from Saudi authorities.

On Thursday, she said the three-member legal and forensic panel would seek to establish "states' and individuals' responsibilities" for the killing.

Khashoggi, a Washington Post columnist living in the United States, was murdered on October 2 at the Saudi consulate in Istanbul where he had gone to collect documents for his planned marriage.

US intelligence agencies believe Crown Prince Mohammed bin Salman (MBS) ordered an operation to kill Khashoggi, who was a critic of his policies, and say his body was dismembered and removed to a location still publicly unknown.

Riyadh denies the crown prince had any involvement in the murder.

"I have requested access to the Saudi consulate in Istanbul and a meeting with the ambassador of the Kingdom of Saud Arabia in Turkey," Callamard said in an email to Reuters news agency. "I have also sought permission to conduct a similar country-visit to the Kingdom of Saudi Arabia."

The Saudi mission in Geneva and government media office in Riyadh have not responded to Reuters' inquiries about the international investigation.

A Saudi public prosecutor's spokesperson said last year that 21 Saudis had been taken into custody over the case, 11 of whom had been indicted and referred to trial.

The prosecutor said earlier this month his office was seeking the death penalty for five of the 11 indicted suspects.

"I conceive of this inquiry to be a necessary step, among a number of others, towards crucial truth telling about and formal accountability for the gruesome killing of Mr Khashoggi," Callamard said.

Last Thursday, Callamard said that a legal and forensics team comprising three international experts would try to find out "the nature and extent of the responsibilities of states and individuals" in relation to Khashoggi's killing.

In its report to the UN Human Rights Council in June, the inquiry will make recommendations about how to ensure "formal (criminal) accountability", she said.

Callamard said she had requested information from other authorities, including in the US.

"It is hoped this will help ensure accountability and transparency in this case and may open new avenues for the prevention and protection of the right to life in other cases, including of journalists and human rights defenders, and accountability for their killings," she added.

Turkey wants Saudi Arabia to extradite those accused of carrying out the murder to be tried in Turkish courts.

Late last year, Turkey briefed the US on the findings of the Turkish public prosecutor's investigation, repeatedly confirming that Khashoggi's murder had been ordered by the highest levels of the Saudi authorities and that the Saudi assassination team had not acted on its own.