A documentary that raises the controversial topic of sexual assault in the US military has caused major policy changes before its official release.

The film, The Invisible War directed by Kirby Dick, reveals that there have been more than 500,000 rapes reported in the US army, with female soldiers being more likely to suffer a sexual assault from within their own ranks than be hit by enemy fire.

Its title refers to the victim's struggle to have their complaints heard and investigated by an organisation keen to protect its public image.

Victims of sexual assault interviewed in the film describe how they have been marginalised and even threatened while they sought punishment for their attackers.

The film has already made an impact, with secretary of defence Leon Panetta being so struck by an April screening that he announced improvements in the method of investigating rapes in the military.

The policy changes aim to take the decision whether to move forward with an investigation out of the hands of a unit commander, aiming to cut out the practice of protecting the soldier accused.

Instead the decision on how to proceed will be made by a colonel, or a captain in the Navy.

Kirby told movie blog The Wrap that the film aimed to make policy changes, but "we just didn't think it would happen this soon".

While he is pleased with the changes, he said they were insufficient, since the power to decide whether to proceed with a case continues to rest within a military chain of command.

According to the film's findings, only eight percent of sexual assault cases are prosecuted in the military, with just two percent leading to a conviction.

The Invisible War was positively received at the Sundance film festival in January, where it won the audience award for best documentary.

The film-makers have now set their sights on reaching the highest levels of the US government. Dozens of private screenings were set up for audiences ranging from retired generals to veterans groups and active-duty officers.

The Invisible War will be on general release from 22 June.