TWELVE people have been killed in a gun attack at a midnight screening of the latest Batman movie in the US.

A gunman in a gas mask opened fire at the cinema in the Denver suburb of Aurora and set off a smoke or tear gas canister, local radio said.

Reporter Brenda Stuart said movie-goers thought the attack was part of the film at first.

She told Sky News: ''A lot of people thought the gunshots were part of the movie.

''Then bullets started flying.''

She added: ''Police officers are taking people to hospital themselves, not waiting for ambulances.''

Ms Stuart said the incident happened about 30 minutes into the screening of The Dark Knight Rises.

''Right now they have no suspects,'' she said. Local TV pictures showed police cars surrounding the shopping mall where the Century 16 cinema is located.

According to reports, two gunmen wearing body armour may have been involved in the attack.

One has been arrested and a manhunt is under way for the second, according to reports.

Local radio station KOA said initial police estimates were of 30-40 injured.

Aurora police spokeswoman Cassidee Carlson said: ''The scene is still very active and we have little information for release at this time.''

Nicole Williams, a spokeswoman at the Swedish Medical Centre in the town, said two people injured at the cinema had arrived at the hospital in critical condition.

She said emergency workers were reporting that there could be several more patients.