"We will continue to fight aggressively against requirements for companies to weaken the security of their systems," a Facebook representative was cited by Reuters as saying on Thursday.

"These demands would create a chilly precedent and obstruct companies' efforts to secure their products," the spokesperson added.

The social media giant's public support of Apple follows similar backing by the likes of Google, Twitter and WhatsApp against a court order to help crack the encryption on an iPhone belonging to San Bernardino gunman Syed Rizwan Farook.

On December 2, Farook and his wife killed 14 people in an attack at an office Christmas party, which officials say was inspired by the so-called "Islamic State" (IS) militant group.

Google chief executive Sundar Pichai said the case "could be a troubling precedent" and warned that "forcing companies to enable hacking could compromise users' privacy."

FBI adamant

But two of New York's top law enforcement officers accused Apple of being irresponsible by not allowing investigators to hack into the phone.

Farook was one of two shooters in the December 2 attack

The FBI wants the tech giant to disable a feature that wipes the phone's data after 10 failed password entries. Police intend to use the software to try all password possibilities until it finds the right one.

Manhattan District Attorney Cyrus R. Vance Jr. said the case was "the most visible example of how Silicon Valley's decisions are thwarting criminal investigations and impeding public safety."

Vance's office currently holds 175 seized iPhones that remain inaccessible despite court orders allowing prosecutors to search the devices.

Apple has until Tuesday to file a protest to the decision by a magistrate in California.

The company's chief executive Tim Cook said the demand is dangerous and an overreach of government power.

Analysts said Apple is likely to seek to invoke free speech protection measures as one of its key legal arguments to block the order.

The company's defiance has led to a wide variety of supporting and opposing comments on social media, with some people calling for users to #boycottapple.

mm/cmk (AP, dpa, Reuters)