The US has determined that Saudi Arabia's de facto leader - Crown Prince Mohammed bin Salman - has immunity from a lawsuit filed by murdered journalist Jamal Khashoggi's fiancée.

Mr Khashoggi, a prominent Saudi critic, was murdered at the Saudi consulate in Istanbul in October 2018. US intelligence has said it believes Prince Mohammed ordered the killing.

But in court filings, the US State department said he has immunity due to his new role as Saudi prime minister.

Mr Khashoggi's ex-fiancée, Hatice Cengiz, wrote on Twitter that "Jamal died again today" with the ruling.

She, with the human rights group Democracy for the Arab World Now (Dawn), founded by Mr Khashoggi - had been seeking unspecified damages in the US from the crown prince for her fiancée's murder.

Prince Mohammed was named crown prince by his father, King Salman bin Abdulaziz Al Saud, in 2017. The 37-year-old was then handed the role of prime minister in September this year.

Justice Department lawyers said that as "the sitting head of a foreign government," the crown prince "enjoys head of state immunity from the jurisdiction of US courts as a result of that office."

Saudi Arabia said the former Washington Post journalist had been killed in a "rogue operation" by a team of agents sent to persuade him to return to the kingdom.

However, US officials said the CIA had concluded, "with a medium to high degree of certainty", that MBS - as the prince is known - was complicit.

The murder caused a global uproar and damaged the image of Prince Mohammed and his country. He denies any role in the killing of Mr Khashoggi.