A secret video showing US air crew falsely claiming to have encountered a firefight in Baghdad and then laughing at the dead after launching an air strike that killed a dozen people, including two Iraqis working for Reuters news agency, was revealed by Wikileaks today.

The footage of the July 2007 attack was made public in a move that will further anger the Pentagon, which has drawn up a report identifying the whistleblower website as a threat to national security. The US defence department was embarrassed when that confidential report appeared on the Wikileaks site last month alongside a slew of military documents.

The release of the video from Baghdad also comes shortly after the US military admitted that its special forces attempted to cover up the killings of three Afghan women in a raid in February by digging the bullets out of their bodies.

The newly released video of the Baghdad attacks was recorded on one of two Apache helicopters hunting for insurgents on 12 July 2007. Among the dead were a 22-year-old Reuters photographer, Namir Noor-Eldeen, and his driver, Saeed Chmagh, 40. The Pentagon blocked an attempt by Reuters to obtain the video through a freedom of information request. Wikileaks director Julian Assange said his organisation had to break through encryption by the military to view it.

In the recording, the helicopter crews can be heard discussing the scene on the street below. One American claims to have spotted six people with AK-47s and one with a rocket-propelled grenade. It is unclear if some of the men are armed but Noor-Eldeen can be seen with a camera. Chmagh is talking on his mobile phone.

One of the helicopter crew is then heard saying that one of the group is shooting. But the video shows there is no shooting or even pointing of weapons. The men are standing around, apparently unperturbed.

The lead helicopter, using the moniker Crazyhorse, opens fire. "Hahaha. I hit 'em," shouts one of the American crew. Another responds a little later: "Oh yeah, look at those dead bastards."

One of the men on the ground, believed to be Chmagh, is seen wounded and trying to crawl to safety. One of the helicopter crew is heard wishing for the man to reach for a gun, even though there is none visible nearby, so he has the pretext for opening fire: "All you gotta do is pick up a weapon." A van draws up next to the wounded man and Iraqis climb out. They are unarmed and start to carry the victim to the vehicle in what would appear to be an attempt to get him to hospital. One of the helicopters opens fire with armour-piercing shells. "Look at that. Right through the windshield," says one of the crew. Another responds with a laugh.

Sitting behind the windscreen were two children who were wounded.

After ground forces arrive and the children are discovered, the American air crew blame the Iraqis. "Well it's their fault for bringing kids in to a battle," says one. "That's right," says another.

Initially the US military said that all the dead were insurgents. Then it claimed the helicopters reacted to an active firefight. Assange said that the video demonstrated that neither claim was true.

"Why would anyone be so relaxed with two Apaches if someone was carrying an RPG and that person was an enemy of the United States?" he said. "The behaviour of the pilots is like a computer game. When Saeed is crawling, clearly unable to do anything, their response is: come on buddy, we want to kill you, just pick up a weapon ... It appears to be a desire to get a higher score, or a higher number of kills."

Wikileaks says it will shortly release a second secret US military video showing the deaths of civilians in an attack in Afghanistan. The Pentagon has been seeking ways to prevent classified material appearing on Wikileaks, including through "criminal sanctions". Wikileaks has made public classified US army reports on weapons, military units and battle strategy in Iraq and Afghanistan.

The Pentagon report, reflecting the depth of paranoia about where Wikileaks is obtaining its material, speculates that the CIA may be responsible. But perhaps most embarrassing leak for the US defence department was that of the 2008 report itself which appeared on the Wikileaks site last month.