The "brutal and premeditated killing" of Saudi journalist Jamal Khashoggi was "planned and perpetrated by officials of the state of Saudi Arabia," according to the United Nations special rapporteur on extrajudicial, summary or arbitrary executions.

"The murder of Jamal Khashoggi and the sheer brutality of it has brought irreversible tragedy to his loved ones," said Agnes Callamard in the initial findings of her independent investigation, which were released on Thursday.

Khashoggi, a Washington Post columnist who wrote critically about Saudi Crown Prince Mohammed bin Salman, was killed inside the Saudi Consulate in Istanbul on Oct. 2.

Callamard's final report will be delivered to the UN Human Rights Council in June.

"The human rights inquiry I have committed to undertake is a necessary step, among a number of others, towards crucial truth telling and formal accountability," Callamard said.

Her team was given access to parts of the audio recording obtained by Turkey's intelligence agency, but said they were unable to do a "deep technical examination" of the material.

Callamard also has requested an official visit to Saudi Arabia due to "major concerns" regarding the fairness of proceedings for the 11 people facing trial there for Khashoggi's killing.

Reporting at the end of a week-long mission with her team of three experts to Turkey, she said Khashoggi's death was part of a "well-evidenced" global pattern of killing journalists and others critical of regimes worldwide.

"Fleeing abroad in search of safety has become less and less a reliable form of protection," Callamard said. "The international community must take a strong and collective stand against these practices."

Turkish officials have been frustrated by what they say is a lack of co-operation by Riyadh and have called for an international inquiry.

After denying for several weeks that Khashoggi was killed in the consulate, Saudi Arabia indicted 11 people in the killing and is seeking the death penalty against five of them.

Turkey says Saudi Arabia must reveal the identity of whoever who is thought to have disposed of the journalist's body and hold all involved accountable.