Johnny Depp has said text messages he sent about burning and drowning his former partner Amber Heard were a joke based on a Monty Python sketch.

Testifying before a Virginia court on Monday, the actor said he was "ashamed" of the messages, and that they were an attempt at humour.

Mr Depp, 58, sued Ms Heard, 36, over a story she wrote in which she described herself as a victim of domestic violence. He denies any abuse.

Ms Heard filed a $100m counterclaim.

Taking the stand for a fourth day, Mr Depp responded to questions about a 2013 text exchange with British actor Paul Bettany.

"Let's burn her," Mr Depp had wrote. "Let's drown her before we burn her."

He then made a further obscene suggestion "to make sure she is dead".

Asked about the messages in court, Mr Depp said the messages were "directly" from a Monty Python sketch about burning and drowning witches.

"This is a film we'd all watch when we were 10 - it's just irreverent and abstract humour," he said.

During cross-examination, Mr Depp was pressed on his claims that Ms Heard was wrong to portray him as a domestic abuser.

Jurors heard a series of audio recordings of conversations between the former couple, in which Mr Depp could be heard shouting vulgar insults and obscenities at Ms Heard.

In one exchange, Ms Heard shouts at Mr Depp to put his "cigarettes out on someone else". Mr Depp could be heard insulting Ms Heard about her weight.

He grimaced while the clips were played, while Ms Heard seemed to hold back tears.

And lawyers for Ms Heard focused again on Mr Depp's his history of substance use, claiming he would be violent during alcohol and drug binges.

But Mr Depp said: "If anyone had a problem with my drinking, at any time in my life, it was me. The only person I've ever abused in my life is myself."

At times, Mr Depp appeared visibly agitated by the lines of questioning, occasionally snapping at Ms Heard's lawyer, J Benjamin Rottenborn.

Mr Rottenborn introduced a series of negative articles about Mr Depp, some dating back to 2014, trying to demonstrate that damage to his reputation - and his career - had been done years before Ms Heard publicly accused Mr Depp of abuse.

"These are all hit pieces. These are dreck [rubbish]," Mr Depp said.

At the centre of the case is Ms Heard's 2018 opinion piece in the Washington Post, where she describes herself as a "public figure representing domestic abuse".

Mr Depp says the article - which does not mention him by name - is defamatory and derailed his career.

At the trial - now in its third week - lawyers for Mr Depp have claimed Ms Heard was the aggressor, playing the "role" of victim to benefit her career.

Earlier, the jury heard evidence presented by the celebrity ex-couple's former therapist, who described how Mr Depp and Ms Heard engaged in "mutual abuse", as well as from medical workers who treated Mr Depp as he was detoxing himself from opiates.

Several high profile witnesses, including the entrepreneur Elon Musk and the actors James Franco and Paul Bettany, are to appear later in the trial, which is expected to last at least another month.