Zeid Ra'ad Al Hussein's intervention came after Apple's largest rivals backed the tech giant's bid to resist the US government demand seeking to access the iPhone used by one of the attackers in a deadly rampage in San Bernardino, California in December.
"In order to address a security-related issue related to encryption in one case, the authorities risk unlocking a Pandora's Box that could have extremely damaging implications for the human rights of many millions of people, including their physical and financial security," Zeid said in a
He warned that the FBI order would "set a precedent that may make it impossible for Apple or any other major international IT company to safeguard their clients' privacy anywhere in the world".
The FBI wants to unlocked the iPhone used by Syed Farook, who was behind the San Bernardino massacre along with his wife Tashfeen Malik that left 14 people dead.
The agency has argued that by introducing encryption that can lock data, making it accessible only to the user, Apple and other tech companies are essentially creating "warrant-proof zones" for criminals and others that will cripple law enforcement and jeopardise public security.
Apple has in return said that the only way to unlock the handset would be to introduce a weakened operating system, which could potentially leak out and be exploited by hackers and foreign governments.