Apple is battling the US government over accessing locked devices in at least 10 cases around the country, in addition to the iPhone of one of the San Bernardino attackers, court documents show.

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.

US officials argue the case would not set a legal precedent, but Apple and its supporters claim it could force the company to do the same in other cases and lead to a weakening of security.

In the Brooklyn case, prosecutors responded to the Apple letter with their own filing, claiming that the company's position has been "inconsistent at best."

The letter from US Attorney Robert Capers said that "numerous judges around the nation have found it appropriate, under the All Writs Act, to require Apple to assist in accessing a passcode-locked Apple device where law enforcement agents have obtained a warrant to search that device."

Apple's letter to the Brooklyn judge cited eight additional cases in New York, California, Illinois and Massachusetts where the government was seeking assistance in accessing iPhones or iPads.