The US Justice Department is appealing over a New York judge's ruling that Apple cannot be forced to provide the government with access to locked iPhone data.
The filing in Brooklyn federal court on Monday came a week after Judge James Orenstein issued his decision in a routine drug case.
The New York case tackles the same legal ground as the California case in which a judge ordered Apple to create software to help the US break into an encrypted iPhone used by the shooter who killed 14 people in San Bernardino, California, on December 2.
Apple has vocally opposed the government's move. Its pushback has fuelled a national debate over digital privacy rights and national security.
The government's response in the California case is due on Thursday.
