The existence of other court disputes lends credence to Apple's argument that the high-profile legal case in California is about more than a single iPhone.

Apple provided a list of cases where it is opposing the US Justice Department's requests in a February 17 letter to a federal judge in Brooklyn, where the company is challenging government efforts to access an iPhone in a drug trafficking case.

The letter said all the requests sought Apple's assistance under the All Writs Act, a 1789 law which allows the courts broad authority to help law enforcement.

"Apple has not agreed to perform any services on the devices to which those requests are directed," Apple's lawyer Marc Zwillinger said in the letter.

The letter said the cases were 'similar in nature' but did not provide specifics about the government's requests. It said the San Bernardino case was 'even more burdensome' than the other requests because it would require the company to create new software to help investigators break into the iPhone.

Apple has been locked in a legal and public relations battle with the government in the California case, where the FBI is seeking technical assistance in hacking the iPhone of Syed Farook, a US citizen, who with his Pakistani wife Tashfeen Malik in December gunned down 14 people.

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