Apple fired anew Tuesday at the US government's legal fight to force it to break into an attacker's iPhone, saying the tactic would "appall" the country's founders.

Apple dug into its legal position in a written filing ahead of a hearing set for March 22 before a federal judge in Southern California.

Apple stuck to its argument that the FBI was overstepping legal bounds by using an All Writs Act to compel the company to help break an iPhone used by one of the shooters in the December terror attack in San Bernardino, California.

"The government attempts to rewrite history by portraying the Act as an all-powerful magic wand rather than the limited procedural tool it is," Apple attorneys said in a filing that responded to one submitted to the court a week earlier by the Justice Department.

"Thus, according to the government, short of kidnapping or breaking an express law, the courts can order private parties to do virtually anything the Justice Department and FBI can dream up. The founders would be appalled."

Apple urged the court to reject the FBI request on the ground it is forbidden by the Constitution.

- 'Modest' request? -

Forcing Apple to help unlock an iPhone is a "modest" demand that may turn up vital evidence in a terrorist attack, the US government argued in a brief filed last week, upping the ante in its legal standoff with the technology giant.

Apple, which is backed by a broad coalition of powerful rival technology firms and activists, argues that the FBI is seeking a "back door" into all iPhones as part of the probe.

The government brief, in sharp contrast, argued it is a single case of technical assistance in an important national security investigation.

"The court's order is modest," Justice Department lawyers wrote.

"It applies to a single iPhone and it allows Apple to decide the least burdensome means of complying."

An FBI victory in the case could serve as a legal precedent backing requests for access to iPhones by law enforcement agencies throughout the US.

Apple general counsel Bruce Sewell last week slammed the Justice Department brief as reading "like an indictment" and apparently crafted to smear the iPhone maker with innuendo such as implying a "sinister" relationship with China.

He bashed the "cheap shot" brief as "an unsubstantiated effort to vilify Apple" that was on a flimsy legal footing.

Apple attorneys said that the California-based company has "categorically and absolutely not" been asked by any government other than the United States to build a backdoor into a product.

The government brief said the request is similar to requiring telephone companies to install wiretaps under court orders.

Apple is "fully capable of complying with the court's order," government lawyers wrote.