Cage and Stone have begun filming in New York

Much of the film, starring Nicolas Cage, will be made in Los Angeles, but scenes have been shot in New York.

Cage plays one of two policeman who were rescued from the rubble of the World Trade Center 22 hours after they collapsed.

The movie centres on John McLoughlin and William J Jimeno, believed to be the last two survivors brought out.

"We're not doing everyone's story that day," said co-producer Michael Shamberg.

"We're trusted with the accuracy of the particular story that we're telling."

'Love scenes?'

Meetings were held with New York residents and families of the victims to ensure it was handled in a sensitive way.

It was decided to make the majority of it away from the city on a Los Angeles soundstage.

The collapse of the Twin Towers will not be recreated, with only news footage of it being shown on television screens in background.

"We're not doing the 'Towering Inferno-Titanic' version," said Mr Shamberg.

Some family members have expressed concerns over the content of the movie.

"Are there going to be love scenes in it? How do you portray it correctly?" said Lee Ielpi, who lost his firefighter son in the tragedy.

"It has to be done with some reverence."

Stone has previously called the untitled project "a work of collective passion, a serious meditation on what happened, and carries within a compassion that heals".

The film is scheduled to open on 11 August, one month before the attacks' fifth anniversary.