UN investigators carrying out the probe of the murder of journalist Jamal Khashoggi arrived in Istanbul on Monday and will seek to enter Saudi Arabia's consulate where he was dismembered by a "kill team".

Agnes Callamard, the UN special rapporteur on executions, is leading the investigation and begins a week-long mission to Turkey at the government's invitation.

Callamard arrived with a forensic and legal team and met Foreign Minister Mevlut Cavusoglu on Monday. She will meet Istanbul's chief prosecutor on Tuesday, the prosecutor's office said.

Callamard said last week she hadn't yet received a reply from the Saudi authorities over her request to enter the consulate. The team also wants to visit Saudi Arabia as part of the investigation.

Khashoggi, a Washington Post columnist who was living in the United States, was killed on October 2 at the consulate where he had gone to collect documents for his planned wedding. His body is yet to be found.

'Gruesome and brutal'

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US intelligence agencies believe Saudi Crown Prince Mohammed bin Salman (MBS) ordered an operation to assassinate Khashoggi, a government critic. His body was dismembered and taken to an unknown location.

Riyadh denies the crown prince had any involvement in the murder. Saudi authorities have not responded to Callamard's request for access to the consulate.

During her visit, the UN special rapporteur is scheduled to meet Turkish Foreign Minister Mevlut Cavusoglu and Turkish prosecutors carrying out the local investigation into the murder.

"Callamard's investigation is significant because she said she launched it on her own accord, since no authority in the UN or any member state, to her understanding, are demanding an independent inquiry at the moment," Al Jazeera's Stefanie Dekker, reporting from Istanbul, said.

"The crime is so gruesome and brutal that she decided to start an investigation."

11 suspects 'indicted'

A Saudi public prosecutor's spokesperson said 11 suspects had been indicted and referred to trial, with five facing the death penalty.

The UN rapporteur praised the Saudi probe.

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

UN investigators will try to find out "the nature and extent of the responsibilities of states and individuals" in relation to Khashoggi's killing, she added.

Evidence from other governments, including the US, has been requested, said Callamard.

Turkey wants Saudi Arabia to extradite those accused of carrying out the murder to be tried in Turkish courts, something Riyadh has balked at.

Late last year, Turkey briefed the US on the findings of the Turkish public prosecutor's investigation, repeatedly stating Khashoggi's murder had been ordered at the highest levels of the Saudi government.