Within minutes of the announcement of Academy Award nominations on Thursday, up popped a Twitter hashtag to frame a fresh debate about the lack of diversity in Hollywood: #OscarsSoWhite. Before long, it became the social network's top US trending topic.

The slate for the 87th Academy Awards was a reminder of the glacial pace of change in Hollywood's film industry, even after what looked like progress for black actors and filmmakers last year stemming from the best picture winner, "12 Years a Slave".

All 20 actors nominated in the four acting categories this year are white and no women are nominated for either best director or screenwriter. Award watchers called it "the whitest Oscars" in years.

The Academy of Motion Picture Arts and Sciences has some 6,000 members, who are selected for the quality of their work and recommendations by existing members. Academy branches, such as for actors and directors, nominate for their categories, and everyone can nominate best picture contenders.

"The Academy is about 90 per cent white and 70 per cent male and we're seeing the sad result of that in voting," said Tom O'Neil, founder of awards tracker Gold Derby, referring to figures from a 2012 Los Angeles Times study on Academy voters.

Race and gender are not considered, although behind-the-scenes, members say there are debates at branch level about how to make membership more diverse.

David Oyelowo, the star of "Selma," and the film's director Ava DuVernay, both failed to garner nominations despite having been nominated for Golden Globes for their parts in the movie about African-American civil rights activist Martin Luther King Jr. DuVernay made history as the first black woman to be nominated for a Golden Globe best director award.

Some historians had said the film misrepresented President Lyndon Johnson's stand on voting rights, but critics were quick to point out that "Selma" was only the latest historical picture to draw scrutiny over its accuracy.

The film scored a best picture Oscar nomination, and Academy president Cheryl Boone Isaacs - who herself made history as the first black female president of the organisation - drew attention to that.

"I am extremely happy to note that 'Selma' is up for best picture, which means the talent that it took to bring 'Selma' to the screen was recognized, and I think that's important," she said.

But Selma's exclusion in all the other key Oscar races and in the director, producer, actor and writer guild awards, is likely to hurt its chances at winning best picture on Oscars night, said O'Neil, the awards tracker.

"Critics proclaimed it's the best movie of the year and the Oscars shunned it in most categories, so that means something's wrong," he said.

Last year "12 Years a Slave" made history as the first film by a black director, Steve McQueen, to win best picture. African-American John Ridley won best adapted screenplay and Lupita Nyong'o won best supporting actress.

But if race is a big part of the debate, so is gender.

All of this year's best picture nominees - "American Sniper," "Birdman," "Boyhood," "The Grand Budapest Hotel," "The Imitation Game," "Selma," "The Theory of Everything" and "Whiplash" - were male-driven stories with male-dominated casts.