By Dustin Volz and Julia Edwards

WASHINGTON/LOS ANGELES (Reuters) - The U.S. Department of Justice filed a motion seeking to compel Apple Inc (AAPL.O) to comply with a judge's order for the company to unlock the encrypted iPhone belonging to one of the San Bernardino shooters, portraying the tech giant's refusal as a "marketing strategy."

The filing escalated a showdown between the Obama administration and Silicon Valley over security and privacy that ignited earlier this week.

The Federal Bureau of Investigation is seeking the tech giant's help to access the shooter's phone by disabling some of its passcode protections. The company so far has pushed back, and on Thursday won three extra days to respond to the order.

Apple did not immediately respond to a request for comment.

The motion to compel Apple to comply did not carry specific penalties for the company, and the Justice Department declined to comment on what recourse it was willing to seek. In the order, prosecutors acknowledged that the filing "is not legally necessary."

But the Justice Department said the motion was in response to Apple CEO Tim Cook's public statement Wednesday, which included a refusal to "hack our own users and undermine decades of security advancements that protect our customers."

The clash between Apple and the Justice Department has driven straight to the heart of a long-running debate over how much law enforcement and intelligence officials should be able to monitor digital communications.

A federal court hearing in California has been scheduled for March 22 in the case, according to Thom Mrozek, a spokesman for the U.S. Attorney’s Office for the Central District of California.

"Rather than assist the effort to fully investigate a deadly terrorist attack ... Apple has responded by publicly repudiating that order," prosecutors wrote in the Friday order.

"Apple’s current refusal to comply with the court’s order, despite the technical feasibility of doing so, instead appears to be based on its concern for its business model and public brand marketing strategy,” prosecutors said.

The two sides have been on a collision course since Apple and Google began offering default end-to-end encryption on their devices in 2014, a move prompted in part by the surveillance revelations from former National Security Agency contractor Edward Snowden.

But the Justice Department struggled to find a compelling case where encryption proved to be an insurmountable hurdle for its investigators until the Dec. 2 shooting rampage by Rizwan Farook and his wife in San Bernardino, California, which killed 14. Authorities believe the couple was inspired by the Islamic State.

Some technology experts and privacy advocates backing Apple suggest Farook's work phone likely contains little data of value. They have accused the Justice Department of choreographing the case to achieve a broader goal of gaining support for legislation or a legal precedent that would force companies to crack their encryption for investigators.

The case has quickly become a topic in the U.S. presidential race. Republican frontrunner Donald Trump on Friday called for a "boycott" against Apple until the company complied with the court order.





(Reporting by Julia Edwards, Dustin Volz and Lisa Richwine; Additional reporting by David Ingram; Editing by Andrew Hay and Bill Rigby)