A UN rapporteur probing the murder in Turkey of Saudi journalist Jamal Khashoggi said Thursday the killing was planned and carried out by Saudi officials.

Khashoggi, a Washington Post contributor and Saudi regime critic, was murdered at the ultra-conservative kingdom's consulate in Istanbul on October 2.

"Evidence collected during my mission to Turkey shows prima facie case that Mr Khashoggi was the victim of a brutal and premeditated killing, planned and perpetrated by officials of the state of Saudi Arabia," Agnes Callamard said at the end of a visit to Turkey, according to a UN statement.

Turkey says he was killed by a team of 15 Saudis who strangled him at the mission, and media reports have said his body was cut up and dissolved in acid.

After denying the killing for two weeks Riyadh finally described it as a "rogue" operation and arrested a number of senior Saudi officials.

Khashoggi's body has still not been recovered.

"The murder of Jamal Khashoggi and the sheer brutality of it has brought irreversible tragedy to his loved ones," said Callamard, the UN special rapporteur on extrajudicial, summary or arbitrary executions.

"It is also raising a number of international implications which demand the urgent attention of the international community including the United Nations."