US flags at half-mast after shooting

US President Barack Obama has ordered US flags to be flown at half-mast for six days to honour the victims of the deadly shooting at the premiere of a Batman movie in Colorado.

"As a mark of respect for the victims of the senseless acts of violence... I hereby order that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds, at all military posts and naval stations, and on all naval vessels... until sunset, July 25," Obama said in a presidential proclamation.

US flags will also be flown at half-staff at US embassies, consular offices and other facilities abroad, including all military facilities and naval vessels and stations.

At least 12 people were killed and around 40 others wounded in the midnight rampage at a movie theatre in the Denver area showing The Dark Knight Rises.

Obama dropped a planned re-election campaign event in Fort Myers, Florida and instead spoke there about the shooting spree, saying the killings were "beyond reason".

He cut his appearance short and headed to the airport to board Air Force One back to the White House.

His campaign team said they were pulling all contrast ads - commercials contrasting Obama with his Republican rival Mitt Romney - in Colorado, a key battleground state ahead of November's election.

Romney's team said it was taking similar action.

PHOTOS: View images from the scene of the mass shooting in Colorado

Photo: A flag at half-mast at the White House (Getty Images)