Voice assistants, including Apple's Siri and Amazon's Alexa, can be controlled by hackers using inaudible voice commands, researchers at Zhejiang University in China have found.

This can be done using a technique that translates voice commands into ultrasonic frequencies that are too high for the human ear to recognise.

The technique, named DolphinAttack, could be used to download a virus, send fake messages and even add fake events to a calendar.

It could also give hackers access to outgoing video or phone calls, allowing them to spy on their victims.

Voice assistant apps across almost every platform are at risk of being controlled, the study found.

While the commands may go unheard by humans, the low-frequency audio commands can be picked up, recovered and then interpreted by speech recognition systems.