Will Smith has apologised to Chris Rock after he slapped him at the Oscars, saying his behaviour was "unacceptable and inexcusable".

"I would like to publicly apologise to you, Chris," he said in a statement. "I was out of line and I was wrong."

It comes after the Oscars film academy condemned Smith over the incident and announced a formal review.

The star slapped Rock in the face on stage after the comic made a joke about the actor's wife, Jada Pinkett Smith.

Rock had taken aim at Pinkett Smith's shaved head, a result of the hair-loss condition alopecia.

Moments later Smith picked up the first Oscar of his career for playing the father of tennis legends Venus and Serena Williams in King Richard.

"Violence in all of its forms is poisonous and destructive," Smith said in the statement, which was posted to Instagram.

"My behaviour at last night's Academy Awards was unacceptable and inexcusable. Jokes at my expense are a part of the job, but a joke about Jada's medical condition was too much for me to bear and I reacted emotionally."

Smith apologised directly to Rock, stating he was "out of line". He also apologised to the Academy and the Williams family.

"I deeply regret that my behaviour has stained what has been an otherwise gorgeous journey for all of us," he wrote.

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Prior to Smith's apology, the organisation behind the Oscars said in a statement that it "condemns the actions" of Smith.

"We have officially started a formal review around the incident and will explore further action and consequences in accordance with our Bylaws, Standards of Conduct and California law," it said.

Rock's joke referred to the 1997 film GI Jane, in which Demi Moore played the title role with a severe buzzcut.

Pinkett Smith rolled her eyes at the comment, while Smith appeared to initially laugh and clap his hands before he was seen on stage, walking up to Rock.

The comedian looked stunned in the immediate aftermath of the incident, but told the audience: "That was the greatest night in the history of television."

He is yet to comment further on what happened.

The Los Angeles Police Department later told Variety that Rock had "declined to file a police report" following the event.

After the incident, Rock handed over the best documentary prize, which was the reason he was on stage.

Accepting the Oscar for best actor for King Richard soon after, Smith said Richard Williams was a "fierce defender of his family" and that he himself was "being called on" in his life "to love people and to protect people".

"Art imitates life," he said. "I look like the crazy father, just like they said about Richard Williams. But love will make you do crazy things."

He ended it saying he hoped the Academy would invite him back.