Istanbul's chief prosecutor has filed warrants for the arrests of a senior aide to Saudi Arabia's royal family and the deputy head of its foreign intelligence on suspicion of planning the murder of writer Jamal Khashoggi, two Turkish officials said on Wednesday.

The prosecutor's office has concluded there is "strong suspicion" that Saud al-Qahtani and General Ahmed al-Asiri, both removed from their positions in October, were among the planners of Khashoggi's Oct 2 killing at the Saudi consulate in Istanbul, the officials said.

The move came a day after senior US senators said they were more certain than ever that Saudi Crown Prince Mohammed bin Salman was responsible for the killing, citing a CIA briefing.

A group of the US senators introduced a resolution on Wednesday urging the US government to hold the prince accountable for a number of actions, including the Khashoggi killing and contributing to the humanitarian crisis in Yemen.

The United Nations human rights chief on Wednesday called for an international investigation.

Turkish President Tayyip Erdogan has pushed to keep international attention on the murder－the order for which he said came from the highest levels of the Saudi government－even as US President Donald Trump has said Washington should not take action that would undermine its relationship with the kingdom.

"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 Turkish officials 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."

Erdogan has said the order for Khashoggi's killing probably did not come from King Salman, putting the spotlight instead on Salman's heir.

Saudi Arabia has said 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.

Washington-Riyadh ties

Making some of their strongest accusations so far, both US Republicans and Democrats said they want to pass legislation to send a message to Saudi Arabia that the United States condemns the death of Khashoggi, a Washington Post columnist.

Trump and some Republicans have argued that Washington should not take action that would risk its relationship with Riyadh, which is viewed as an important counterweight to Iran in the Middle East.

The US last month imposed economic sanctions on 17 Saudi officials for their role in the killing.

AFP and AP contributed to this story.