WASHINGTON (AFP) - A broad array of technology firms joined Apple's legal fight over encryption on Thursday, warning of a dangerous precedent if the company is forced to help the government break into a locked iPhone.

Three tech associations representing Apple's main business rivals - including Google, Facebook, Microsoft and Yahoo - announced a joint brief supporting Apple's efforts to challenge an order that would require it to help unlock an iPhone used by one of the San Bernardino attackers.

"If the government arguments prevail, the Internet ecosystem will be weakened, leaving Internet users more vulnerable to hackers and other bad actors," said a statement from the Computer and Communications Industry Association (CCIA), which announced a joint amicus brief with the Internet Association and the i2 Coalition of Internet infrastructure firms.

The three associations said their brief was set to be filed before the midnight deadline in federal court in California, where the case is being heard.

A number of other companies and associations are also expected to file briefs in the case, which has divided the American public and set off a highly charged debate about what limits should be placed on law enforcement access to digital devices.

"There is broad and deep concern throughout many types of companies throughout the tech industry that there is a potentially dangerous precedent in this case," said Ed Black, president and chief executive of CCIA.

"While the tech industry understands the government's desire for information, and respects its mission to keep us safe, we hope the court appropriately weighs the wider issues of security and trust that are also at stake in this case."

The case stems from the FBI's efforts to access the locked iPhone used by one of the perpetrators of the December attack in San Bernardino, California, that killed 14 people.

Apple has argued that the only way to unlock the phone would be to introduce a weakened operating system, which could potentially leak out and be exploited by hackers and foreign governments.

The FBI has argued that by introducing encryption that can lock data, making it accessible only to the user, Apple and others are essentially creating "warrant-proof zones" for criminals and others that will cripple law enforcement and jeopardize public safety.

The CCIA includes Apple rivals such as Amazon, Pandora and Samsung, and the Internet Association counts as members Dropbox, Microsoft, Yahoo and Facebook. Some firms are members of both. The i2 Coalition includes Google, which is a member of all three organisations.

Twitter, meanwhile, filed another legal brief supporting Apple, together with 16 other tech firms including eBay, LinkedIn, Airbnb and Reddit.

"The government seeks unbounded authority to compel Apple to design software that does not currently exist and that will circumvent and undermine security measures intended to protect its users' data," the brief said.

This could "set a precedent that could be used in future cases to require (other tech firms) to provide technical assistance in a manner that undermines the very products they offer."

Intel announced its own legal filing supporting Apple, saying that it would be "an unprecedented step for the government to require a company to develop technology that weakens security in a commercial product."

"Such a move chills innovation," the chipmaker said in a statement.

A group of cryptographic experts also filed arguments in support of Apple's case on Thursday.

"The security bypass this court would order Apple to create almost certainly will be used on other iPhones in the future," said the brief filed by the Stanford Law School Center for Internet and Society on behalf of several academic and private sector cryptographers.

"This spread increases the risk that the forensic software will escape Apple's control either through theft, embezzlement, or order of another court, including a foreign government."

Digital rights groups also backed Apple, with the Electronic Frontier Foundation arguing in its brief that the order would violate Apple's free speech rights by forcing it to write weakened software guaranteed with its digital signature.

It would be "akin to the government dictating a letter endorsing its preferred position and forcing Apple to transcribe it and sign its unique and forgery-proof name at the bottom," EFF argued in its brief endorsed by 46 technology specialists.