WASHINGTON (AFP) - Apple fired back at the US government on Thursday in the encryption standoff, asking a federal court to dismiss an order that would force the company to help unlock an iPhone.

In the latest development in the case stemming from last year's San Bernardino attacks, Apple said in a court filing that the government overstepped its legal authority in trying to force the company to facilitate access to the locked iPhone used by one of the shooters.

"No court has ever authorised what the government now seeks, no law supports such unlimited and sweeping use of the judicial process, and the constitution forbids it," Apple's lawyers wrote in the motion filed in California federal court.

The Apple response is the latest in the deadlock between the company and US law enforcement over how far the company must go in helping access a device with data locked by encryption that only the user can normally access.

"The government demands that Apple create a back door to defeat the encryption on the iPhone, making its users' most confidential and personal information vulnerable to hackers, identity thieves, hostile foreign agents, and unwarranted government surveillance," Apple's brief said.

Apple said it designed the security "so that when customers use an iPhone, they can feel confident that their most private personal information" and that any access provided to government would weaken that security.

Apple executives, who briefed reporters on condition they not be quoted directly, said the order would effectively require the creation of "a new operating system" or "government OS" which could be used repeatedly by FBI forensics experts and potentially leak out to others.

On this basis, the iPhone maker is arguing that the government effort violates Apple's constitutional rights of free speech, by forcing it to write software which undermines its values.

Apple's brief also said the legal showdown came despite a pledge by government officials not to seek legislation for easier access to encrypted devices.

"If anything, the question whether companies like Apple should be compelled to create a backdoor to their own operating systems to assist law enforcement is a political question, not a legal one," the Apple filing said.

The filing comes one week after the US Justice Department filed a motion to compel Apple to provide the "reasonable technical assistance" sought by the FBI.

The government filing, in sharp contrast to that of Apple, said the order would not require a "back door to every iPhone."

The Justice Department said that Apple's public statements suggest it is basing its defence on "marketing concerns" and that the company was not being asked to hand over any sensitive software that could be used by hackers.

Earlier on Thursday, FBI director James Comey reiterated his position at a hearing on Capitol Hill.

"The San Bernardino litigation is not about us trying to send a message or establish some kind of precedent," Comey told lawmakers at the House Intelligence Committee.

"It's about trying to be competent in investigating something that is an active investigation."

The phone at the centre of the standoff belonged to Syed Farook, a US citizen, who carried out the attack on an office party in San Bernardino along with his Pakistani wife Tashfeen Malik.

Separately, US lawmakers moved to break the deadlock by calling a hearing with the FBI and Apple in an effort to craft "a solution" to the dilemma of accessing locked devices.

A hearing called by the House Judiciary Committee for next Tuesday will be the first in Congress since Apple said it would challenge a court order to help unlock an iPhone used by one of the San Bernardino attackers.

"Our goal is to find a solution that allows law enforcement to effectively enforce the law without harming the competitiveness of US encryption providers or the privacy protections of US citizens," said a statement from Judiciary chairman Bob Goodlatte, a Republican from Virginia, and ranking Democrat John Conyers of Michigan.

The panel has already held a briefing from technology companies and a classified briefing from the government on the issue.

A statement said scheduled witnesses included FBI chief Comey, and Bruce Sewell, Apple's senior vice president and general counsel.

Also scheduled to appear were Susan Landau, a cybersecurity expert at Worcester Polytechnic Institute and Cyrus Vance Jr, the New York district attorney who has criticised Apple for locking its iPhones without allowing access for law enforcement.

The debate has divided the public as the FBI seeks to get Apple's assistance in unlocking an iPhone used by one of the perpetrators of the December attack in San Bernardino, California that killed 14.