PAWLEYS ISLAND, South Carolina (AFP/REUTERS) - Republican presidential frontrunner Donald Trump called on Friday for a boycott of Apple until the computer giant complies with US government demands to unlock the iPhone of a suspect in the San Bernardino attacks.

"Apple ought to give the security for that phone, OK?" Trump told a campaign rally in Pawleys Island, South Carolina, one day before the state holds its hotly contested Republican primary.

"What I think you ought to do is boycott Apple until such time as they give that security number," he said. "I just thought of it. Boycott Apple."

The Republican front-runner in the 2016 White House race has been a frequent critic of Apple and called on the company to make more products in the United States.

Trump's spokeswoman, Hope Hicks, said the real estate mogul does not use an iPhone.

His latest comments came as the US Department of Justice filed a motion seeking to force Apple to comply with a judge's order for the company to unlock the iPhone belonging to one of the San Bernardino shooters, portraying the tech company's refusal as a "marketing strategy."

Apple had pledged this week to fight a magistrate's order to assist in unlocking the phone used by one of the shooters.

"First of all, the phone's not even owned by this young thug that killed all these people. The phone's owned by the government," Trump said.

Suspect Syed Farook, who was killed in a shootout with authorities after the attack that left 14 dead, worked for the county in California where the December rampage occurred.

Trump lashed out at Apple chief executive Tim Cook, saying he was "looking to do a big number, probably to show how liberal he is."

The motion brought by the Justice Department sought to debunk Apple's claim that cooperating with the FBI probe would undermine overall security for its devices, and laid out the legal case for technical assistance.