Intelligence agencies in the United States believe Saudi Arabia had tried to spy on slain journalist Jamal Khashoggi's fiancee when she visited the United Kingdom last year.

UK-based The Guardian newspaper on Friday reported that the US agencies asked their UK counterparts to "keep a close eye" on Hatice Cengiz, 38, after they learned of the Saudi plan to surveil her.

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According to the report, US officials believed Riyadh had the "ambition and intention" to monitor Cengiz in London last May, months after Khashoggi was killed in October 2018 inside the Saudi consulate in Istanbul.

"The Guardian's revelations about the effort by US and British authorities to ensure Cengiz was protected follows a report by the Washington Post columnist David Ignatius that the US state department recently rejected a proposal by a US defence company to train Saudi intelligence services," said the report.

It added that the proposal was rejected since Saudi Arabia did not have "proper safeguards in place to prevent lawless covert operations" such as Khashoggi's killing.

The Guardian said the revelations "highlight the concerns of human rights activists" who allege Riyadh is using surveillance to "monitor and intimidate dissidents and critics of the kingdom".

The report said it was not confirmed if the surveillance of Cengiz was electronic or physical, or whether the plot was successful.

The Guardian report came days after claims that a mobile phone belonging to Jeff Bezos, the billionaire owner of the Washington Post, was hacked after he received a WhatsApp message from the personal account of Saudi Crown Prince Mohammed bin Salman (MBS).

A digital forensic analysis suggested the theft of data from the phone of Amazon's boss in 2018 started with an infected video file sent via WhatsApp.

It's unclear whether the alleged hack of Bezos's phone accessed any sensitive corporate information. Saudi Arabia has categorically denied the kingdom was involved in the hacking.

The murder of Saudi columnist Jamal Khashoggi inside Saudi consulate had shocked the world [File: Hasan Jamali/AP]

Khashoggi, a critic of MBS, used to write an opinion column for the Post, commenting on Saudi foreign policy and the repression of free speech in the kingdom, with his writings often taking a direct aim at the crown prince.

The CIA reportedly concluded that Khashoggi's killing was ordered by the Saudi crown prince, but he has denied the allegations. Last month, a Saudi court sentenced five people to death for the murder.