* Saudi prince may have been involved in alleged hack - UN experts
* They call for immediate investigation by U.S. officials, others
* Study found malware probably sent via WhatsApp - UN experts
* Alleged hack said to have come months before Khashoggi murder (Adds U.N. experts on spyware, details)
By Joseph Menn and Raphael Satter
SAN FRANCISCO/WASHINGTON, Jan 22 (Reuters) - U.N. experts have demanded an immediate investigation by U.S. and other authorities into allegations that Saudi Arabia's crown prince was involved in a plot to hack the phone of Amazon boss Jeff Bezos.
The U.N. special rapporteurs, Agnes Callamard and David Kaye, said on Wednesday that they had information pointing to the "possible involvement" of Crown Prince Mohammed bin Salman in the alleged 2018 cyberattack.
Saudi officials dismissed the allegations as absurd.
Callamard, the special rapporteur for extra-judicial killings, and Kaye, special rapporteur for free expression, said the allegation of Saudi involvement "demands immediate investigation by U.S. and other relevant authorities".
They said a forensic analysis of the alleged hacking, which a person familiar with the matter said had been commissioned by Bezos, concluded his phone was probably hijacked by a malicious video file sent from a WhatsApp account purportedly belonging to the crown prince in April or May of 2018.
The rapporteurs said the analysis, which they deemed credible, found that within hours of receiving the video there was "an anomalous and extreme change" in the device's behavior, with the level of outgoing data from the phone jumping nearly 300-fold.
The allegations could further damage relations between billionaire tech tycoon Bezos and Riyadh, and risk harming the kingdom's reputation with foreign powers and investors.
The alleged cyberattack is said to have taken place months before the October 2018 murder of Saudi journalist Jamal Khashoggi, a columnist for the Bezos-owned Washington Post and a critic of the crown prince.
The CIA believed that Prince Mohammed ordered the killing of Khashoggi, sources told Reuters here weeks later. He was murdered in the Saudi consulate in Istanbul by Saudi agents and his body dismembered.
Prince Mohammed, or MbS, said last year that the killing was carried out by rogue operatives and that he did not order it.
Saudi Foreign Minister Prince Faisal bin Farhan Al Saud dismissed the allegations of the prince's involvement in any phone hack of Bezos.
"I think 'absurd' is exactly the right word," he told Reuters in an interview in Davos. "The idea that the crown prince would hack Jeff Bezos’ phone is absolutely silly."
The special rapporteurs, who released their findings in a statement, report to the U.N. Human Rights Council but they are independent watchdogs and not U.N. officials.
Their recommendations are not binding on countries, though are widely seen as carrying moral weight.
They stopped short of identifying which specific technology might have been used in the alleged hack, but said software like that made by Israeli company NSO Group or Italian spyware maker Hacking Team could potentially have been deployed.
NSO denied its technology was used in the alleged hack.
"We know this because of how our software works and our technology cannot be used on U.S. phone numbers," it said. "Our products are only used to investigate terror and serious crime."
After a merger last year, Hacking Team is now part of Swiss-Italian cyber intelligence firm Memento Labs. Memento Labs' head, Paolo Lezzi, was not immediately available to comment but has previously said he has no knowledge of Hacking Team's former operations.
In another previous flashpoint between the Amazon founder and Riyadh, Bezos' security chief said last year that the Saudi government had gained access to his phone and leaked messages to U.S. tabloid the National Enquirer between Bezos and Lauren Sanchez, an ex-TV anchor who the newspaper said he was dating.
A month before, Bezos had accused the newspaper's owner of trying to blackmail him with the threat of publishing "intimate photos" he allegedly sent to Sanchez.
The Saudi government has denied having anything to do with the National Enquirer reporting.
The Guardian first reported here the crown prince's alleged involvement in a phone hacking plot on Bezos.
Saudi Arabia's U.S. embassy also dismissed the allegations.
"We call for an investigation on these claims so that we can have all the facts out," it said in a message posted on Twitter.
Amazon declined to comment. (Additional reporting by Hesham Abdul Khalek in Cairo, Jeffrey Dastin in San Francisco and Kevin Krolicki in Davos and Raphael; Writing by Pravin Char; Editing by Mike Collett-White)