U.S. President Donald Trump has issued fresh criticism towards tech giant Apple.

He tweeted: “We are helping Apple all of the time on TRADE and so many other issues, and yet they refuse to unlock phones used by killers, drug dealers and other violent criminal elements. They will have to step up to the plate and help our great Country, NOW! MAKE AMERICA GREAT AGAIN.”

On Monday U.S. Attorney General William Barr said the firm was being unhelpful in an inquiry into a shooting that is being treated as a terrorist act.

Mr Barr noted that Apple had failed to provide "substantive assistance" to unlock two iPhones in an investigation into a fatal shooting at a naval base in Pensacola, Florida. Three U.S. sailors were killed when a Saudi trainee at the base opened fire on 6 December.

Apple has refuted the claims: “Our responses to their many requests since the attack have been timely, thorough and are ongoing," it said in a statement.

According to its website, the company regularly provides information from its servers to law enforcement when it is subpoenaed. Apple has responded to over 127,000 requests for information from law enforcement since 2013. An Apple privacy official said earlier this month that the company has departments established to respond to legal authorities’ requests.

Tim Cook, Apple’s CEO, has spent years fostering a friendly relationship with the current White House hoping to influence the President against tariffs in the U.S.-China trade war, which would hurt Apple because its products are primarily assembled in China.  It avoided tariffs due to the Phase-One pact set to be signed by the two countries on Wednesday.

Read the latest news update: Malaysia Airlines halts 737 MAX delivery