The murder of journalist Jamal Khashoggi was "planned and perpetrated" by officials from Saudi Arabia, according to a UN expert investigating the killing.

Mr Khashoggi - a prominent critic of the Saudi regime, including powerful crown prince and de facto leader Mohammed bin Salman - disappeared after entering the Saudi consulate in Istanbul on October 2nd.

After earlier denials, Saudi Arabia ultimately confirmed he was murdered in the building in Turkey.

The government has claimed the killing was a 'rogue operation', and insisted members of the royal family had nothing to do with it.

Saudi prosecutors have said they are seeking the death penalty for five people accused of being involved in the murder.

Now, the UN Special Rapporteur leading an independent human rights inquiry into the journalist's death has delivered her initial findings.

"Seriously curtailed and undermined"

United Nations Special Rapporteur Agnes Callamard, surrounded by members of the media walks around the Saudi Consulate, background, in Istanbul on January 29th. Picture by: Cemal Yurttas/AP/Press Association Images

According to the findings, the killing "violated both international law and core rules of international relations" - including regarding the rules for diplomatic missions.

The inquiry also found that Turkey's effort to investigate the killed were "seriously curtailed and undermined" by Saudi Arabia.

Agnes Callamard - the UN Special Rapporteur on extrajudicial, summary or arbitrary executions - said Turkish investigators were given "woefully inadequate time and access" to carry out a crime-scene examination.

She explained: "Evidence collected during my mission to Turkey shows prime 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.

"The murder of Jamal Khashoggi and the sheer brutality of it has brought irreversible tragedy to his loved ones. It is also raising a number of international implications which demand the urgent attention of the international community including the United Nations."

The investigation is ongoing, with Callamard's team yet to meet with chief investigators and specialists due to time constraints and other factors.

A final report is due to be presented to the UN in June.