US Secretary of Defense Ashton Carter has said he opposes high-tech "back doors" that would allow the government access to encrypted data on people's phones and other devices.

The Pentagon chief's views come amid a legal battle between Apple and the FBI, which is trying to force the tech giant to help unlock an iPhone used by one of the shooters in the San Bernardino attacks last December.

"Just to cut to the chase, I'm not a believer in back doors or a single technical approach to what is a complex and complicated problem," Carter said to applause at a tech event in San Francisco on Wednesday, according to a transcript.

"There isn't going to be one answer," Carter added.

"I don't think we ought to let one case drive a general conclusion or solution. ... We have to work together to work our way through this problem."

The FBI has said it does not want a back door, but needs Apple's help cracking the iPhone's passcode.

Apple has argued that the FBI is effectively asking the company to hack its own devices and create a back door that malicious actors could exploit, and many in the tech industry worry the case would lead to a slew of similar requests.

Apple's refusal to help the FBI has set off an intense political debate about encrypted devices.

The iPhone in question 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.