A US magistrate judge in New York has ruled that police overstepped the law when they called on Apple to unlock the iPhone of a suspected crystal meth dealer Jun Feng, whose case is ongoing though Feng has pleaded guilty.

Federal Judge James Orenstein said in a 50-page opinion that law enforcement lacked the authority to compel Apple to comply.

"The relief the government seeks is unavailable because Congress has considered legislation that would achieve the same result but has not adopted it," he wrote.

He also said that US Justice Department cannot use a 227-year-old law to force Apple Inc to provide the FBI with access to locked iPhone data, dealing a blow to the government in its battle with the company over privacy and public safety.

The government sought access to the phone in the Brooklyn case in October, months before a judge in California ordered Apple to take special measures to give the government access to the phone used by one of the shooters in the San Bernardino, California, attacks.

His ruling echoed many of the arguments that Apple has made in the San Bernardino case, particularly his finding that a 1789 law called the All Writs Act cannot be used to force Apple to open the phone.

"This is precisely on point in the San Bernardino case," a senior Apple executive said during a telephone briefing with reporters after the New York ruling was issued.

The judge found that Apple was largely exempt from complying with such requests by a 1994 law that updated wiretapping laws.

"As attacks on our customers' data become increasingly sophisticated, the tools we use to defend against them must get stronger too," he said.

"Weakening encryption will only hurt consumers and other well-meaning users who rely on companies like Apple to protect their personal information."

Encryption helps preserve privacy around the world, he added, "and it keeps people safe."

Conflicting rulings on the same points of law would increase the chances of the US Supreme Court being called upon to make the final call.