SAN FRANCISCO:Facebook and Twitter are siding with Apple in its fight against a court order requiring the company to help investigators break into an iPhone used by one of the San Bernardino mass shooters.

A United States has magistrate ordered Apple to produce software that would give investigators access to the iPhone at issue.

Apple has until Tuesday to challenge the order, setting the stage for a legal clash that experts say could change the relationship between tech companies and government authorities in the US and around the world.

Twitter's chief executive Jack Dorsey tweeted that the microblogging site stands with Apple Inc. and its CEO Tim Cook and thanked Cook for his leadership.

Facebook in a statement said it condemns terrorism and also appreciates the essential work of law enforcement in keeping people safe. But it said it will 'fight aggressively' against requirements for companies to weaken the security of their systems.

“These demands would create a chilling precedent and obstruct companies efforts to secure their products,” the statement said.

The government isn't asking Apple to help break the iPhone's encryption directly, but to disable other security measures that prevent attempts to guess the phone's passcode.

Cook argues that once such a tool is available, the technique could be used over and over again, on any number of devices, even as law enforcement insists that safeguards could be employed to limit its use to that particular phone.

He has posted a 1,117-word open letter on how the FBI's request might have implications far beyond the legal case at hand.

For months, Cook has engaged in a sharp, public debate with government officials over his company's decision to shield the data of iPhone users with strong encryption - essentially locking up people's photos, text messages and other data so securely that even Apple can't get at it.

Law-enforcement officials from FBI Director James Comey on down have complained that terrorists and criminals may use that encryption as a shield.

“This is really a deep question about the power of government to redesign products that we use,” said Ryan Calo, a University of Washington law professor who studies data security and privacy issues.

While other tech companies have spoken against broad government surveillance in the past, the Obama administration has recently sought to enlist the tech industry's help in fighting terrorism.

Several companies have recently heeded the administration's request for voluntary efforts aimed at countering terrorist postings on social media.

Civil liberties groups warned the fallout from the San Bernardino dispute could extend beyond Apple.

"This is asking a company to build a digital defect, a design flaw, into their products,” said Nuala O'Connor of the Center for Democracy and Technology, a Washington-based group that has criticised government surveillance. In a statement, the center warned that other companies could face similar orders in the future.

Others said a government victory could encourage regimes in China and other countries to make similar requests for access to smartphone data. Apple sells millions of iPhones in China, which has become the company's second-largest market.

“This case is going to affect everyone's privacy and security around the world,” said Lee Tien, a staff attorney for the Electronic Frontier Foundation, a digital rights group in San Francisco.

The case turns on an 18th-century law that the government has invoked to require private assistance with law enforcement efforts.

Apple has also challenged a federal search warrant based on the same law in a Brooklyn drug case.

Apple has complied with previous orders invoking that law - the All Writs Act of 1789 - although it has argued the circumstances were different.

While experts said the case will likely end up in appeals court, both sides seemed to be framing the debate for a public audience as much as for a judge.

"The federal request is very strategic on their part, to be sure” said Robert Cattanach, a former Justice Department lawyer who handles cyber-security cases for the Dorsey & Whitney law firm.

He said it appeared the government took pains to ask only for limited assistance in a mass-murder case that horrified the nation.

Apple's Cook, however, declared the demand would create what amounts to a “backdoor” in Apple's encryption software.

“If the government can use the All Writs Act to make it easier to unlock your iPhone, it would have the power to reach into anyone's device to capture their data,” he wrote in the open letter. Cook also pledged respect for law enforcement and outrage over the shootings.

Cook may have no choice but to mount a legal challenge, given his very public commitment to protecting customer data.

Two fellows at the Brookings Institution criticised that stance Thursday, writing that Apple's “self-presentation as crusading on behalf of the privacy of its customers is largely self-congratulatory nonsense.”

Cook has made privacy protection a part of Apple's marketing strategy, drawing a contrast with companies like Google and Facebook that sell advertising based on customers' online behavior.

"Apple can't be seen now as doing something that would make their products less safe,” said Wendy Patrick, who lectures about business ethics at San Diego State University. “I think everyone saw this issue coming down the pike and Apple always knew it was going to push back when the moment came.”

But in doing so, Apple risks alienating consumers who put a higher value on national security than privacy.

A recent survey by the Pew Research Center found 82 percent of US adults deemed government surveillance of suspected terrorists to be acceptable. Apple's stance was already drawing fire Wednesday from GOP presidential candidate Donald Trump and commentators on Fox News.

Only 40 per cent of the Pew respondents said it's acceptable for the government to monitor US citizens, however.

The survey also found nearly three-fourths of US adults consider it very important to be in control over who can retrieve personal information about them.