Independent UN rights experts said Wednesday they had received information that Amazon owner Jeff Bezos's phone was hacked through a WhatsApp account belonging to Saudi Crown Prince Mohammad bin Salman.
The allegations have been rejected by Riyadh, whose embassy in Washington branded them as "absurd".
"The alleged hacking of Mr Bezos's phone, and those of others, demands immediate investigation by US and other relevant authorities," UN Special Rapporteurs Agnes Callamard and David Kaye said in a statement in Geneva.
They said any investigation into the alleged incident in May 2018 should also look at the "continuous, multi-year, direct and personal involvement of the Crown Prince in efforts to target perceived opponents".
Callamard, the UN expert on summary executions and extrajudicial killings, and Kaye, the expert on freedom of expression, said they were "gravely concerned".
"The information we have received suggests the possible involvement of the Crown Prince in surveillance of Mr Bezos, in an effort to influence, if not silence, The Washington Post's reporting on Saudi Arabia," they wrote.
Bezos is the owner of The Washington Post, which employed as a contributing columnist Jamal Khashoggi, a Saudi journalist murdered in October 2018 at Riyadh's consulate in Istanbul.
"Recent media reports that suggest the Kingdom is behind a hacking of Mr Jeff Bezos' phone are absurd," the Saudi Arabian embassy said on its Twitter account.
"We call for an investigation on these claims so that we can have all the facts out."
The UN Special Rapporteurs said the circumstances and timing of the hacking incident also gave grounds for further investigation into "allegations that the Crown Prince ordered, incited, or, at a minimum, was aware of planning for" the operation to kill Khashoggi.
- 'Unprecedented exfiltration' of data -
The two experts said they had become aware of a 2019 examination of Bezos's iPhone that found it may have been hacked on May 1, 2018 with an MP4 video file sent from an account used by the Saudi Crown Prince.
The two had exchanged numbers a month before, they said.
The analysis reportedly found that within hours of receiving the video file, Bezos's phone saw an "unprecedented exfiltration" of 126 MB of data.
This continued undetected over a period of "some months" with rates of as much as 4.6 GB higher than the baseline.
The forensic analysis cited by the UN experts showed that the Crown Prince, Saudi Arabia's de factor ruler, sent WhatsApp messages to Bezos in November 2018 and February 2019 in which he revealed information about Bezos's personal life that was not available from public sources.
The analysis also suggested that the hackers may have used a type of spyware used in other Saudi surveillance cases, such as the NSO Group's Pegasus-3 malware.