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 on suspicion of planning the killing of Saudi journalist Jamal Khashoggi.

The prosecutor's office concluded that there is "strong suspicion" that Saud al-Qahtani and Ahmed Asiri, who were both removed from their positions following the murder, were among the planners of the murder, two Turkish officials said on Wednesday.

Jamal Khashoggi, a United States resident and columnist for the Washington Post, was killed shortly after entering the Saudi consulate in Istanbul on October 2.

"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," one of the officials told Reuters news agency.

The official added that Saudi Arabia could address the international concern by extraditing all suspects in the case to Turkey.

According to AFP news agency, the application for the warrants was filed on Tuesday.

At the time of publication, Saudi Arabia had not publicly responded to the request.

The Saudi prosecution has previously acknowledged that al-Qahtani and Asiri were part of the plot to kill Khashoggi.

The two men were both high-ranking officials with close ties to Crown Prince Mohammed bin Salman.

Al-Qahtani is one of the highest-profile figures implicated in the killing. Believed to have been Prince Mohammed's right-hand man.

The 40-year-old was removed as a royal court adviser following Khashoggi's assassination. Prior to that, he served as a media adviser to Prince Mohammed.

Al-Qahtani is believed to have supervised a 15-man hit squad that flew from the Saudi capital, Riyadh, to Istanbul to carry out Khashoggi's murder, although he did not travel to Turkey.

Nor did Asiri, who served as spokesperson for the Saudi-led coalition in Yemen before being appointed as an adviser to Prince Mohammed, who then promoted him to his intelligence position in 2017.

Asiri frequently was the subject of condemnation from rights groups over apparent disregard for civilian casualties in the war in Yemen.

Al Jazeera's Mohamed Vall, reporting from Istanbul, said it is possible that more arrests will be ordered.

"The Turkish prosecution believes that these men are only a part of the planning and we understand that the list is not conclusive.

"We understand also that there was a previous request from the Turks to Saudi Arabia to extradite the 18 men they mentioned were involved in the crime but none of that has happened.

"No response came from Saudi Arabia and now there is this specific mention of these two men at the top, but it's not a conclusive list and they said these men are among the planners, not all the planners," he said.

US pressure needed

Wednesday's announcement came hours after CIA director Gina Haspel briefed US senators on new evidence in the Khashoggi case.

Republican Senator Lindsey Graham said he felt there was "zero chance" the crown prince wasn't involved in Khashoggi's death.

While several countries have taken steps to put pressure on the kingdom, including in some cases suspending weapons exports, it is felt that Saudi Arabia is unlikely to comply with Turkish demands without encouragement from the US.

"We know that Mohammed bin Salman takes his cues from President Trump and that Trump so far has not commented on the statements that have been issued during the night from Congress members.

"I think the Saudis on their own will not comply with any Turkish demands unless there is enough pressure put on them from the American side," Valls said.

Also on Wednesday, United Nations human rights chief Michelle Bachelet said that an international investigation was needed to determine who was responsible for Khashoggi's murder.

"I do believe it is really needed in terms of ensuring what really happened and who are the [people] responsible for that awful killing," she said at a news conference in Geneva.

Speaking in Brussels on Wednesday, Turkish Foreign Minister Mevlut Cavusoglu urged Saudi Arabia to be transparent, saying Ankara would not hesitate to launch an international probe if its investigation becomes deadlocked.

Khashoggi's murder prompted international outcry and forced many countries to reassess their ties with the kingdom.

After weeks of repeated denials that it had any involvement with his disappearance, Riyadh eventually acknowledged that Saudi officials had planned and executed the killing.

The whereabouts of his remains are still unknown.

Agencies