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.
Riyadh has rejected the allegations, its embassy in Washington branding them "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.
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", they added.
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.
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Bezos owns 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."