The Turkish fiancee of slain Saudi journalist Jamal Khashoggi said on Monday that Saudi crown prince Mohammed bin Salman should be "punished without delay" over the murder.

The United States on Friday declassified a report that publicly accused the crown prince of approving Khashoggi murder in the kingdom's Istanbul consulate in 2018.

But the United States stopped short of applying sanctions against the 35-year-old de facto leader, known by his initials MBS.

"It is essential that the Crown Prince, who ordered the brutal murder of a blameless and innocent person, should be punished without delay," Khashoggi's fiancee Hatice Cengiz said in a statement posted on her official Twitter account in English and Arabic.

"This will not only bring the justice we have been seeking for Jamal, but it could also prevent similar acts recurring in the future," she said.

Khashoggi, a US resident and critic of Prince Mohammed who wrote for The Washington Post, was killed and dismembered at the kingdom's Istanbul consulate after going inside to receive paperwork for his wedding to Cengiz.

Cengiz said that "following this report, there is no longer any political legitimacy for the Crown Prince".

But she said the US report did not go far enough.

"The truth -- that was already known -- has been revealed one more time, and it is now confirmed."

"Yet this is not enough," she warned, "since the truth can only be meaningful when it serves justice being achieved."

Cengiz said if the crown prince was not punished, "it will forever signal that the main culprit can get away with murder which will endanger us all and be a stain on our humanity."

The Saudi government, which initially said it had no information on Khashoggi, says it accepts responsibility for the killing but casts it as a rogue operation that did not involve the prince.

In a statement late Friday, the Saudi foreign ministry said it "completely rejects" the declassified US report.