WASHINGTON (AFP) - Apple's challenge of a court order to unlock an iPhone used by one of the San Bernardino killers opens up a new front in the long-running battle between technology companies and the government over encryption.

The standoff brings the sensitive issue, which has been at a stalemate in Congress, into the courts, which could ultimately impose a decision with wide-ranging consequences.

A California magistrate on Tuesday ordered Apple to provide "reasonable technical assistance" to the US Federal Bureau of Investigation (FBI) to break into an iPhone used by one of the shooters in the deadly December rampage.

Apple quickly said it would fight the judge's order.

Chief executive Tim Cook called it "an unprecedented step which threatens the security of our customers," and said the order "has implications far beyond the legal case at hand."

Apple, Google and other technology firms in recent years have stepped up encryption - allowing only the customers to have "keys" to unlock their devices - claiming improved security and privacy is needed to maintain confidence in the digital world.

That drive for privacy has prompted sharp objections from law enforcement and intelligence officials, who claim that criminals and extremists are able to hide their illicit activities thanks to device encryption.

"This is a clever move by the FBI to move from the legislative arena, where they were not winning, to the courts," said Joseph Hall, chief technologist at the Centre for Democracy and Technology, a digital rights group.

"They appear to be asking for Apple to produce a version of iOS that removes the glue to keep security working."

The order raised hackles among privacy advocates, which see the potential to unleash unbridled surveillance in the United States and elsewhere.

"If the FBI can force Apple to hack into its customers' devices, then so too can every repressive regime in the rest of the world," said Alex Abdo of the American Civil Liberties Union.

"Apple deserves praise for standing up for its right to offer secure devices to all of its customers."

The case is likely to work its way through the courts, which will need to consider a number of both technical and legal questions.

Interestingly, Magistrate Judge Sheri Pym's order is based on the 1789 All Writs Act, which lays out broad authority for the courts to help enforcement of the law.

Legal scholar Jonathan Turley of the George Washington University said Pym "has gone far beyond what I consider the scope of her authority" and that her actions "appear almost legislative in nature."

"Congress has not ordered such back door access to be supplied by companies and such a move would raise difficult privacy questions," Turley said in a blog post.

The order also "raises chilling constitutional questions that should be fully reviewed by a federal district court judge and an appellate court," Turley said.

Berin Szoka, president of the libertarian think-tank TechFreedom, said the order goes against "bedrock principles of law and privacy."

"If forcing Apple to hack its own devices qualifies as 'reasonable technical assistance,' there is no practical limit to what law enforcement could force private companies to do to compromise the security of their systems," Szoka said in a statement.

Darren Hayes, a Pace University professor of computer forensics, argued that Apple and other tech companies may have gone too far by using encryption that, in theory, makes it impossible for the firms to hand over evidence even if served with a legal warrant.

"I think that the public, once they become more educated about what is happening, might change their stance about Apple," said Hayes, who has worked as a consultant to law enforcement.

"This case is sensitive for the US public and I don't think it's particularly good public relations for Apple" to refuse to help the investigation, Hayes added.

A key question is whether Apple has the ability to provide the assistance sought by the FBI.

CDT's Hall said that Apple "has never been forthcoming about the deep details of its system" but noted there is "a lot of well-informed speculation" about its encryption.

"Apple does not have the keys to your device - they are burned onto your chip," Hall said.

But he said it may be possible to get around that encryption.

"They have software that could essentially do things that the keys can do," he said, but noted that it would probably require a weakening of the overall operating system's security.

Hayes said that no one knows for sure if Apple can circumvent the encryption, but noted "the fact that they're arguing this makes me think they may be able to do it."