Apple Is Refusing to Unlock an iPhone 5S for U.S. Law Enforcement The company says that complying with the request will 'threaten the trust between Apple and its customers and substantially tarnish the brand.'

Law enforcement agencies in the US have hit back at Apple for not unlocking an iPhone that could help them solve a criminal investigation they're working on.

The US Justice Department wants Apple to bypass the lock screen on an iPhone in compliance with a search warrant but Apple is refusing to comply.

The case, being held at the District Court's Eastern District of New York, involves an iPhone 5s, according to the Electronic Frontier Foundation.

Technically, Apple has the ability to access data on the iPhone 5s because it is running the company's iOS 7 operating system, which lacks the encryption features that have been built into subsequent versions of Apple's mobile software.

However, Apple is refusing to unlock the phone for the Justice Department on the grounds that it will "threaten the trust between Apple and its customers and substantially tarnish the brand."

Apple wrote in its court filings: "Apple respectfully requests that the court deny the government's application for an order requiring Apple to perform extraction services on the Apple-manufactured device in the government's custody,"

The US Justice Department argues that Apple customers have no rights under the end user license agreement (EULA) – except to use the software in ways approved by Apple.

The government department filed its response to Apple's brief last Thursday, arguing that the All Writs Act, used by law enforcement agencies to get legal access to smartphones, gives the court sufficient authority to compel Apple to unlock the phone.

"Apple has such a close connection to the devices it sells that it can be compelled to step in and take control of the device," the US Justice Department wrote.

Apple has been increasing the level of encryption in its mobile software amid heightened privacy concerns following leaks by former National Security Agency contractor Edward Snowden about NSA surveillance programs.

Apple told courts last week that its ability to bypass lock screens for iPhones running iOS 8 or iOS 9 was limited because it had strengthened encryption methods. Those devices come with a feature that makes it impossible for anyone without the passcode to access its data, including Apple.

This rise of strong encryption techniques has frustrated law enforcement, who fear it means that evidence risks "going dark."