Actress Amber Heard says ex-husband Johnny Depp threatened to kill her.

Amber Heard began giving evidence on Monday in Johnny Depp's libel action against a British tabloid, telling London's High Court that her ex-husband had threatened to kill her.

Depp is suing News Group Newspapers, publishers of the Sun newspaper, over a 2018 article which labelled him a "wife beater" and questioned his casting in the "Fantastic Beasts and Where To Find Them" franchise.

"Some incidents were so severe that I was afraid he was going to kill me, either intentionally or just by losing control and going too far," Heard said in her witness statement.

"He explicitly threatened to kill me many times, especially later in our relationship."

The court has already been told that Heard, 34, accuses her ex-husband of attacking her on at least 14 occasions between 2013 and 2016 when he became enraged after drinking or taking drugs to excess, accusations Depp, 57, has denied.