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US presidential candidate Donald Trump has called for a boycott of Apple until the tech giant helps unlock the iPhone of one of the San Bernardino killers.

Apple has clashed with the Justice Department (DoJ) over a court order forcing the company to help break the encryption on one of its phones.

On Friday the DoJ called Apple's refusal a "marketing strategy".

Apple said it will not help break into the the phone, citing wider privacy concerns for its users.

The phone belonged to one of the two people who opened fire at an office event in San Bernardino, California, last December, killing 14

Speaking at a campaign rally, Mr Trump said: "Boycott Apple until such time as they give that information."

On Thursday, a court ordered the tech giant to help break the encryption.

The government has called the request narrow and argued it is only focused on this particular iPhone.

The DoJ filed another motion in court on Friday after Apple's chief executive, Tim Cook, said Apple would continue to refuse the order.

A California court has set 22 March for the hearing.

Analysis: Dave Lee, BBC North America technology reporter

Before this goes to court, it's being played out with a war of words.

The DoJ motion today is about investigators smashing the ball back into Tim Cook's court. It disputes his claim that the security of its products would be inherently and forever compromised.

It essentially argues that Mr Cook's stance - and indeed the stance of the other technology companies that support him - is motivated by business, not ethics.

Apple has been given an extension to submit its formal response, but the real sparks are likely to fly at the hearing date set for late March.

It will take place in San Bernardino under, you'd imagine, the scrutiny of the families caught up in the tragedy.

In a letter to customers published on Apple's website, Mr Cook called the implications of the order "chilling".

"While we believe the FBI's intentions are good, it would be wrong for the government to force us to build a backdoor into our products. And ultimately, we fear that this demand would undermine the very freedoms and liberty our government is meant to protect."