Oscar-winning actor George Clooney has accused the Academy Awards of "moving in the wrong direction" amid controversy over lack of diversity.
For the second year running, no black or minority actors have been nominated in the four Oscars acting categories.
Star Wars actress Lupita Nyong'o also said she is "disappointed by the lack of inclusion" in the nominations.
Academy president Cheryl Boone Isaacs vowed to review its membership as stars pledged to boycott the ceremony.
'Calling for change'
Clooney, who has won two Academy Awards, said: "We need to get better at this. We used to be better at it."
Nyong'o, who won a best supporting actress Oscar for 12 Years a Slave in 2014, wrote on Instagram that she stands with those "calling for change" and that the awards should be a "diverse reflection of the best of what our art has to offer today".
And British director Steve McQueen, whose 12 Years a Slave won best film, has said black actors need to be given a "fair bite".
Speaking to Variety, Clooney said: "If you think back 10 years ago, the Academy was doing a better job.
"Think about how many more African Americans were nominated. I would also make the argument, I don't think it's a problem of who you're picking as much as it is: How many options are available to minorities in film, particularly in quality films?"
He said African Americans "have a real fair point that the industry isn't representing them well enough", arguing that Creed, Concussion, Beasts of No Nation and Straight Outta Compton should have received more nominations.
"I think around 2004, certainly there were black nominees - like Don Cheadle, Morgan Freeman," said Clooney. "And all of a sudden, you feel like we're moving in the wrong direction. There were nominations left off the table."
He added: "By the way, we're talking about African Americans. For Hispanics, it's even worse. We need to get better at this. We used to be better at it."
Director Spike Lee, who was awarded an honorary Oscar in November, is among those boycotting the ceremony, saying on Instagram that he "cannot support" the "lily white" Oscars.
Jada Pinkett Smith has also refused to attend because of the number of white nominees, saying in a video message on Facebook that people of colour should disregard the awards.
McQueen told the BBC: "I think racism has a lot to do with it, but also the whole idea of people not being adventurous enough in thinking outside of the box as such, of what they possibly think is the norm.
"It can't be about box office, because I think black actors and stories along those lines have been doing very, very well, obviously.
"So it's about executives in cinema and film studios, television, cable networks, giving those storylines and those actors a fair bite."
Selma star David Oyelowo has been vocal about this year's Oscar nominations, saying - in reference to African American Boone Isaacs: "This institution doesn't reflect its president.
"I am an Academy member and it doesn't reflect me. It doesn't reflect this nation."
Boone Isaacs has said she is taking action to "alter the make-up" of their membership, saying: "As many of you know, we have implemented changes to diversify our membership in the last four years. but the change is not coming as fast as we would like. We need to do more, and better and more quickly."
The 6,300 members of the Academy, made up of people from the film industry, vote on who is nominated for the Oscars each year.
Academy member Michael Moore, director of Bowling for Columbine, is among the latest to say he will boycott the 28 February ceremony.
He told The Wrap he supports Boone Isaacs' efforts to implement change, but added: "The idea that we could go two years in a row, where 40 actors could be nominated and none of them were black, is just crazy.
"So if it will help to lend my name to what Spike and Jada are doing, I'm hoping to be a symbolic participant in this [boycott]."
The profile of Oscar voters
In 2012, the LA Times conducted a study to find out how diverse the Academy membership is.
Reporters spoke to thousands of Academy members and their representatives to confirm the identities of more than 5,100 voters - more than 89% of the voting members.
They found that:
- Oscar voters are nearly 94% Caucasian and 77% male. Black members make up about 2% of the Academy, and Latino members less than 2%.
- Oscar voters have a median age of 62. People younger than 50 constitute just 14% of the membership.
- Some of the Academy's 15 branches are almost exclusively white and male. Caucasians currently make up 90% or more of every Academy branch except actors, whose membership is 88% white.
- Men make up more than 90% of five branches, including cinematography and visual effects.
Oscar-winning actor Cuba Gooding Jr has told Variety: "You want it (the Oscars) to be diverse. You want the work to show. I wanted Straight Outta Compton to get something. But, you know, it's this conversation that makes people think harder when the nominations come around for next year."
Will Packer, producer of Straight Outta Compton, posted a long message on Facebook, saying: "To my Academy colleagues, WE HAVE TO DO BETTER. Period.
"The reason the rest of the world looks at us like we have no clue is because in 2016 it's a complete embarrassment to say that the heights of cinematic achievement have only been reached by white people."