Twenty-one members of the Saudi military are being expelled from the US after a cadet carried out a mass shooting at a air base last month.

The servicemen are not accused of aiding the 21-year old Saudi Air Force lieutenant.

But US Attorney General William 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 wounded in the 6 December attack.

Training for Saudi servicemen was put on hold in the US after the attack.

Mr Barr told a news conference on Monday that the shooting at Naval Air Station Pensacola had been an "act of terrorism".

He 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, Mr Barr said, but FBI investigators were able to restore the device.

"We have asked Apple for their help in unlocking the shooter's iPhones," Mr Barr said." So far Apple has not given us any substantive assistance."

Apple had given the FBI iCloud data from the attacker's online account, the New York Times reported, 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.

Mr Barr said that initial reports that other Saudi cadets had filmed the attack as it unfolded were inaccurate. The gunman had arrived at the scene of the shooting alone, he said.