NEW YORK (AFP) - Oscar nominations are three months away but Birdman is already generating rave reviews and awards buzz for actor Michael Keaton, who delivers the performance of his career 20 years after Batman.

A black comedy mixed with magical realism, Birdman tells the story of Riggan Thomson, a washed-up superhero movie star desperate to revive his flagging career by directing and starring in his own Broadway adaptation.

Along the way he grapples with daughter, fresh from rehab and working as his assistant, played by Emma Stone; his mistress and co-star played by Andrea Riseborough; ex-wife Amy Ryan and a hard-drinking, troublesome actor played by Edward Norton.

Directed by Mexican-born Alejandro Gonzalez Inarritu, the film's US premiere closed the New York Film Festival on Saturday and goes on general release across the United States on Friday.

Reviewers hail it one of the best films of the year, and say Keaton delivers an Oscar-worthy performance in what may be his biggest role since starring as Tim Burton's Batman in 1989 and 1992.

"It is a genuine artistic triumph and an honest-to-goodness delight. 'Birdman' is so good that its quality is downright thrilling," enthused Forbes.

Entertainment Weekly tipped it as an Oscar contender for best film, best actor for Keaton, best director for Inarritu, best supporting actor for Norton and best supporting actress for Stone.

Keaton, the original superhero movie star, and for many still the ultimate Batman, was the perfect choice to play Thomson.

Yet the 63-year-old actor, known for his subtlety and comic timing, has been out of the headlines for years and has distanced himself from too much overlap between his real life and onscreen persona.

"In terms of the parallels, I've never related less to a character than Riggan but I did understand him on a lot of levels because he was so visceral and true and heartbreakingly human," he said.

The movie was filmed in New York's St. James Theatre where Keaton's character is putting on an adaptation of Raymond Carver's short story What We Talk About When We Talk About Love.

The cast spent most of the 30-day shoot doing complex unbroken shots that lasted as long as 10 minutes and - thanks to Inarritu and Oscar-winning cinematographer Emmanuel Lubezki - make it seem as though the film was shot in one continuous take.

Naomi Watts, who plays actress Lesley on set, told reporters that it "created this high-level of intensity and pressure that felt emblematic of how it feels on the stage".

"A lot of my nightmares revolve around being on the stage and forgetting my lines or having the wrong clothes on or no clothes at all," she joked, when asked about the pressure of theatre work.

"I've had the same dream that Naomi has of seeing Naomi on stage naked," Keaton joked. "It's not a nightmare, believe me."

As a comedy, however black, "Birdman" is a departure for Inarritu, whose previous films have been almost unbearably tragic.

"I decided after so much spicy food I have had, I wanted dessert and some rest in my tummy," he joked.

But if the approach to his other movies is different, he says the underlying message is the same.

He says he finds it "incredibly tragic" but also "beautiful and funny" that humans fail in their solemn attempts to succeed. "I think that's the only way we as human beings can survive, laughing at ourselves a little and having a little better time."

There are some great lines, such as when Watts' character asks "why don't I have any self-respect?" only for Riseborough's to reply: "You're an actress, honey."

It pokes fun at snobbery in the form of snooty New York Times theatre critic Tabitha, played by Lindsay Duncan, who tells Keaton's character: "You're not an actor, you're a celebrity."

And it mocks the modern definition of success and instant gratification quantified by likes and followers on social media.

But Inarritu said "the DNA of the film" was the Carver short-story about the quest in life to be really loved.

"That is what we human beings are looking for, I think no matter who you are, and I think what Riggan Thomson is looking for: validation, love, affection."