The United Nations’ human rights chief has called for an international investigation to determine who was responsible for the murder of journalist Jamal Khashoggi.

Michelle Bachelet said a probe was “really needed” to find out who was responsible for “that awful killing”.

It comes after Turkey instructed Saudi Arabia to extradite all suspects in the murder, including two key allies of crown prince Mohammad bin Salman.

Istanbul’s chief prosecutor has filed warrants for the arrest of a top aide to Saudi Arabia’s de facto ruler and the deputy head of foreign intelligence, suspected of planning the killing.

The prosecutor’s office has concluded there is “strong suspicion” that Saud al-Qahtani and General Ahmed al-Asiri, who were removed from their positions in October, were among the planners of the 2 October killing at the Saudi consulate in Istanbul, according to two Turkish officials.

One official said: “The prosecution’s move to issue arrest warrants for Asiri and Qahtani reflects the view that the Saudi authorities won’t take formal action against those individuals.”

Turkey has been trying to extradite 18 suspects, including 15 members of the alleged assassination squad. Saudi Arabia has detained 21 people and says it is seeking the death penalty for five.

Jamal Khashoggi death: key figures Show all 7 1 /7 Jamal Khashoggi death: key figures Jamal Khashoggi death: key figures Jamal Khashoggi Washington Post journalist who was critical of the Saudi regime and the young Crown Prince Mohammed bin Salman, he was murdered on 2 October in the Saudi Arabian consulate in Istanbul AFP Jamal Khashoggi death: key figures Saudi Crown Prince Mohammed bin Salman Heir to the Saudi throne, Mohammed bin Salman has been implicated in the murder, with US officials claiming that he must have known of the plot AFP/Getty Jamal Khashoggi death: key figures 15 man hit squad Turkish police suspect these 15 men of being involved in the murder of Jamal Khashoggi, issued 10 October, 8 days after the journalist disappeared EPA Jamal Khashoggi death: key figures Saud al-Qahtani Aide to Crown Prince Mohammed bin Salman, Saud al-Qahtani is claimed to have ordered Khashoggi's murder Saud Al-Qahtani/Twitter Jamal Khashoggi death: key figures Maher Abdulaziz Mutreb A former diplomat who often travelled with the Crown Prince, Mutreb was initially claimed to be the leader of the hit squad and is pictured here entering the Saudi consulate on the day of the murder AP Jamal Khashoggi death: key figures Mustafa al-Madani First implicated in the 15 CCTV photos released by the Turkish police, al-Madani was later found to have been used as a body double for Khashoggi, leaving the Saudi consulate dressed in his clothes on the day the journalist was killed CNN Jamal Khashoggi death: key figures Salah bin Jamal Khashoggi (L) Son of the murdered journalist met with Crown Prince Mohammed bin Salman on 23 October EPA

It comes a day after several top US senators said they were even more convinced Saudi Arabia’s crown prince was involved in the death after leaving a briefing with CIA director Gina Haspel.

Lindsey Graham, who had demanded the briefing with Ms Haspel, said there was “zero chance” the crown prince was not involved.

“There’s not a smoking gun. There’s a smoking saw,” Mr Graham said, referring to reports from the Turkish government, which said Saudi agents used a bone saw to dismember Khashoggi after he was killed.

Mr Graham said “you have to be willfully blind” not to conclude this was orchestrated and organised by those under the crown prince’s command.

Donald Trump has equivocated over who is to blame for the killing, defending the crown prince by denying reports the CIA had concluded he was behind the killing.

Donald Trump defends Saudi Arabia accusation of Jamal Khashoggi murder: 'Do people really want me to give up hundreds of thousands of jobs?'

The Turkish official said: “The international community seems to doubt Saudi Arabia’s commitment to prosecute this heinous crime. By extraditing all suspects to Turkey, where Jamal Khashoggi was killed and dismembered, the Saudi authorities could address those concerns.”

Turkish president Recep Tayyip Erdogan has said the order for Khashoggi’s killing came from the highest level of the Saudi government, but probably not King Salman, putting the spotlight instead on his heir and de facto ruler prince Mohammad.

Saudi Arabia has claimed the prince had no prior knowledge of the murder.

After offering numerous contradictory explanations, Riyadh later said Khashoggi had been killed and his body dismembered when negotiations to persuade him to return to Saudi Arabia failed.