A furious Frank Stallone slammed the Academy in a foul-mouthed tirade Monday for snubbing big brother Sylvester in the best supporting actor category at this year's Oscars.

The Rocky star was widely backed to bag the prize following a Golden Globe award for his critically acclaimed turn as the boxer in Ryan Coogler's spin-off movie Creed.

But the statuette went instead to Britain's Mark Rylance for Bridge of Spies, in one of the biggest shocks of Sunday's gala ceremony at Hollywood's Dolby Theatre.

Livid actor and musician Frank, 65, used Twitter to express his feelings.

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He later apologised to Rylance for the rant, explaining he would always stick up for family.

Stallone himself was in more magnanimous mood, posting a picture of himself on Instagram in his black and blue Oscar tuxedo winking at the camera, fist clenched in defiance.

The 69-year-old was last Oscar-nominated 39 years ago as best lead actor and writer for Rocky.

A host of celebs rushed to ally themselves with Team Sly, including British media personality Piers Morgan, US actress Mia Farrow and Arnold Schwarzenegger, Stallone's co-star in action franchise The Expendables.

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Rylance, 56, was asked backstage how it felt to have outpunched Stallone but said he didn't take awards too seriously.

"I don't know, people come up and say things about competing as actors and I know it's necessary to make a show out of it," he told reporters."I feel more I am a spokesman than someone better than the (other) nominees."

- AAP