A new film depicting the toxic gas leak that killed thousands in India's Bhopal city puts the blame squarely on Union Carbide for the disaster, the director said.

Indian filmmaker Ravi Kumar rejected criticism that "Bhopal: A Prayer for Rain" was too soft on the US company, saying he had told the truth in the film.

The movie opens in Indian cinemas on Friday, just days after the 30th anniversary of the December 2 tragedy, whose victims are still fighting for better compensation.

Starring Hollywood's Martin Sheen and already showing in US cinemas, the movie has been accused of placing greater blame on Indian management at the chemical factory and less on Union Carbide.

Around 3,500 people were killed soon after the factory spewed a cloud of gas over Bhopal, and up to 25,000 died in the years that followed.

Those living near the factory who survived suffered related illnesses, while women gave birth to children with deformities.

"We have tried to tell the events as they unfolded. When the audience comes out of the film there's no doubt who was at fault," Kumar told AFP.

"It's the American Union Carbide Corporation that has the responsibility of the disaster. That's the truth and that is what we have told in the film," he said.

Kumar suggested lessons still needed to be learnt from the tragedy.

"The mechanism for most industrial disasters... is eerily familiar -- cost-cutting, corporate greed, untrained staff and ignorance of early warning signs by the management."

"We want to ensure that accidents such as Bhopal belong in the history books."

The screenplay is based on court evidence, correspondence, testimonies, hospital and forensic records and memories and experiences of survivors and Carbide workers.

Sheen plays Warren Anderson, CEO of Union Carbide, which was taken over by Dow Chemical in 2001.

Kumar said the central fictional character, Dilip, is an amalgamation of the many survivors and victims.

"Our aim was to convey the message of the disaster while entertaining the audience with the thriller drama of the film," he said.

Hollywood's Mischa Barton plays a lifestyle reporter in the film, who helps an Indian journalist portrayed by Kal Penn. Indian actors Rajpal Yadav and Tannishtha Chatterjee star as the victims.