Khashoggi was killed by Saudi agents in the Saudi consulate in Istanbul in an operation which US intelligence believed was ordered by Prince Mohammed, who has been the kingdom’s de facto ruler for several years.

The prince denied ordering Khashoggi’s killing, but acknowledged later it took place “under my watch”.

Last week his elderly father King Salman named him prime minister in a royal decree which a Saudi official said was in line with responsibilities the crown prince was already exercising.

“The Royal Order leaves no doubt that the Crown Prince is entitled to status-based immunity,” lawyers for the prince said in a petition requesting the court dismiss the case, citing other cases where the United States has recognised immunity for a foreign head of state.

US President Joe Biden, who fist-bumped the crown prince on a visit to Saudi Arabia in July to discuss energy and security issues, had told Prince Mohammed that Biden held him responsible for Khashoggi’s killing. He said Prince Mohammed denied involvement and asserted those involved had been held to account.

Khashoggi, who criticised the crown prince’s policies in Washington Post columns, was killed and dismembered in the Saudi consulate in Istanbul. He went there to obtain papers he needed to marry Hatice Cengiz, a Turkish citizen.

The lawsuit was filed jointly by Cengiz and a human rights group founded by Khashoggi, and sought unspecified damages against the crown prince, known in the West as MbS. It also named more than 20 other Saudis as co-defendants.

It charged that MbS, his co-defendants and others carried out a plot to “permanently silence Mr. Khashoggi” after discovering he planned to use the group as “a platform to espouse democratic reform and promote human rights”.

The court had asked the US Department of Justice to express a view on whether Prince Mohammed had immunity, setting an October 3 deadline for a response.

After the prince’s appointment as prime minister last week, the department said on Friday it was seeking a 45-day extension to prepare its response to the court “in light of these changed circumstances”.

(Reporting by Mike Scarcella; Writing by Dominic Evans; Editing by Josie Kao.)