WASHINGTON (AFP) - FBI director James Comey on Thursday defended his agency's efforts to force Apple to help unlock an iPhone used by one of the San Bernardino attackers, as he warned of risks of unbreakable encryption.

"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."

Apple is at the heart of a closely watched legal battle after a US judge ordered the tech giant to provide assistance to the FBI to unlock the encrypted iPhone in question, which belonged to Syed Farook, a US citizen.

Along with his Pakistani wife Tashfeen Malik, Farook gunned down 14 people in the Californian city of San Bernardino in December.

Investigators want help unlocking the device but, Apple chief Tim Cook says unlocking the iPhone in the name of fighting terrorism would be "bad for America".

"It is a precedent that should not be done in this country, or in any country," he told ABC News on Wednesday.

Apple has equated the request to "hacking" its own devices and building a back door which could be used by others.

Comey said the question of whether tech firms should be able to create tamper-proof encryption was the "hardest question" he had ever seen in government.

"It's going to require negotiations and conversation," he said.

Comey noted that while encryption is attractive in principle, law enforcement must have some sort of continued access to information to keep the public safe.

"Law enforcement really saves people lives," he said. "We do that a whole lot through court orders that are search warrants."

Comey told lawmakers the public should understand "the costs associated with moving to a world of universal strong encryption."

He commented that with the growth of this kind of encryption, "the world will not end but it will be a different world from where we are today."

Facebook chief executive officer Mark Zuckerberg has said he is "sympathetic" with Apple's quandary and other tech firms have offered guarded support.

CIA director John Brennan said in an interview broadcast on Wednesday that the FBI "clearly has a legitimate basis" to demand the unlocking of the phone in question.