Trump tweeted from his handle @realDonaldTrump: "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'," Trump wrote.
Trump's remarks came after US Attorney General William Barr on Tuesday declared the Florida naval air base attack by a Saudi cadet as an act of 'jihadist terror'.
Twenty-one members of the Saudi military are being expelled from the US after a cadet carried out a mass shooting at the air base last month.
Barr said the cadets were found to have had jihadist material and indecent images of children in their possession.
Three sailors were killed and eight were wounded in the December 6 attack by a gunman identified as Mohammed Saeed Alshamrani.
Training for Saudi servicemen was put on hold in the US after the attack.
On Monday, Barr said he had asked Apple to unlock two iPhones that belonged to the gunman, who was killed by police in the attack.
The gunman fired a bullet into one phone in an effort to destroy it, Barr said, but FBI investigators were able to restore the device.
"We have asked Apple for their help in unlocking the shooter's iPhones. So far Apple has not given us any substantive assistance," Barr said.
Apple had given FBI iCloud data from the attacker's online account, but refused to unlock the phone, saying it would undermine their own encryption software.
The tech firm has clashed previously with the FBI over requests to unlock iPhones belonging to terror suspects.
A similar 2016 clash was resolved when the FBI found a way to unlock a phone belonging to a mass shooter in California without help from Apple.
( With inputs from IANS )