Apple says that the FBI's demands that it should help them access an encrypted iPhone belonging to one of the San Bernardino killers is unconstitutional and would "set a dangerous precedent", the company has said in court documents.

Investigators at the FBI are seeking Apple's help to access gunman Rizwan Farook's iPhone by disabling some of its passcode protection.

The tech giant has filed papers with a US court claiming that its computer code is protected under the US First Amendment's right to free speech and it can't be "conscripted" to do the government's bidding.

In court documents, they said:

The Justice Department is proposing a "boundless interpretation" of the law that, if left unchecked, could bring disastrous repercussions for digital privacy, the company warned in the memo.

"The government says: 'Just this once' and 'Just this phone.' But the government knows those statements are not true," lawyers for Apple wrote.

Farook,28 and his wife Tashfeen Malik,27, killed 14 people and injured 21 others when they attacked an office Christmas party in California last year.