In this photo released by Saudi Press Agency, SPA, Saudi Crown Prince Mohammed bin Salman, right, shakes hands with Salah Khashoggi, a son, of Jamal Khashoggi, in Riyadh, Saudi Arabia, Tuesday, Oct. 23, 2018. File picture: Saudi Press Agency via AP.

The eldest son of murdered Saudi journalist Jamal Khashoggi, Salah, and his family have left the Gulf kingdom after the government lifted a travel ban, Human Rights Watch said Thursday.

"Salah and his family are on a plane to (Washington) DC now," Sarah Leah Whitson, HRW's executive director for the Middle East and North Africa, told AFP, citing a family friend.

There was no immediate comment from Saudi officials, but Whitson said that they were apparently allowed to leave after a travel ban on Salah was lifted.

Khashoggi, a Washington Post contributor, was killed on October 2 after a visit to the Saudi consulate in Istanbul to obtain paperwork before marrying his Turkish fiancee. 

After first insisting Khashoggi left the consulate unharmed, Saudi authorities said he was killed in an argument that degenerated into a brawl. 

Riyadh finally accepted on Thursday what Turkey had said virtually from the start -- that he was killed in a premeditated hit.

Saudi Arabia has sought to draw a line under the crisis, but has come under mounting pressure from sceptical world powers demanding answers and the whereabouts of Khashoggi's body.

US President Donald Trump has derided the killing as "one of the worst cover-ups" in history.