This image taken from a video shot from a U.S. army Apache helicopter and posted on WikiLeaks.org shows a group of men on a street in eastern Baghdad just prior to being fired upon by the helicopter on July 12, 2007. (Wikileaks.org/Associated Press)

The website WikiLeaks released a leaked classified U.S. military video Monday showing a 2007 helicopter attack that killed a dozen people in Baghdad, including a driver and photographer working for the Reuters news agency.

WikiLeaks, which publishes leaked government, court and other documents that reveal misconduct, said at a news conference in Washington that it acquired encrypted video of the July 12, 2007, U.S. attack from military whistleblowers, Reuters reported.

Its staff broke the encryption code and verified the footage, which comes from the gunsight of one of the two Apache helicopters that carried out the attack.

A U.S. defence official, speaking on condition of anonymity, confirmed that the video and accompanying audio are authentic, Reuters said.

Reuters had been trying to obtain the video evidence through the Freedom of Information Act ever since the attack, which the military investigated at the time, concluding that it conformed to its rules of engagement and that there was no misconduct.

After the attack, the military said its forces were engaged in combat with hostile forces, but no such activity is seen on the video.

Camera mistaken for grenade launcher

The 17-minute excerpt of the gunsight footage posted on WikiLeaks.org, along with accompanying documents, shows an aerial view of several men walking down and milling about the streets of a Baghdad neighbourhood.

The accompanying audio tracks the conversation between pilots and commanders as they begin shooting at the men, believing them to be armed insurgents.

Among the group of men were Reuters photographer Namir Noor-Eldeen, 22, and his assistant and driver, Saeed Chmagh, 40, who were killed in the incident.

The helicopter gunsight tracks two of the men, identified by WikiLeaks as the Reuters news staff, as the pilots mistake their cameras for weapons and start shooting.

Military spokesman Maj. Shawn Turner said the pilots mistook a camera for a rocket-propelled grenade launcher.

The pilots open fire on the small group, killing several people and wounding others.

Shortly after firing, they are heard laughing. One of the pilots is heard saying, "Oh, yeah, look at those dead bastards" as the gunsight pans the carnage on the ground, partly obscured by dust whipped up by the shooting. Another voice replies, "Nice."

'Graphic evidence' of dangers of war journalism

Minutes later, when a van approaches and several men emerge and begin trying to assist the wounded, the U.S. pilots become concerned that the men are trying to collect weapons and help the wounded escape.

They request permission to "engage" and when permission is granted, open fire, killing several people in and around the vehicle and wounding two children inside the van.

Later, as the children are being carried from the scene by U.S. soldiers on the ground, one of the pilots can be heard saying, "Well, it's their fault for bringing their kids into a battle."

As the U.S. military vehicles approach the scene, a voice can be heard saying, "I think they just drove over a body."

David Schlesinger, editor in chief of Reuters, said the video showed that the deaths of Noor-Eldeen and Chmagh were "tragic and emblematic of the extreme dangers that exist in covering war zones."

"The video released today via WikiLeaks is graphic evidence of the dangers involved in war journalism and the tragedies that can result," he said.

Reuters has pressed the U.S. military to conduct a full and objective investigation into the deaths of its two employees.