A US army intelligence analyst was today charged with leaking a highly classified video of American forces killing unarmed civilians in Baghdad and secret diplomatic cables to WikiLeaks.

Private Bradley Manning, who had a top-secret security clearance, has been held in military custody in Kuwait since his arrest in Iraq in May over the video, which caused great embarrassment to the US military establishment. It showed an air strike that killed a dozen people, including two Iraqis working for Reuters news agency. The air crew is heard falsely claiming to have encountered a firefight in Baghdad and then laughing at the dead. WikiLeaks gave the video the title Collateral Murder.

Manning, 22, was arrested after boasting in instant messages and emails to a high-profile former hacker, Adrian Lamo, that he passed the material to WikiLeaks along with thousands of pages of confidential American diplomatic cables.

WikiLeaks has since said it plans to release a second US military video that shows one of the deadliest US air strikes in Afghanistan, in which scores of children were believed to have been killed. The site's founder, Julian Assange, said the organisation was still working to prepare the encrypted film of the bombing of the Afghan village of Garani in May 2009, in which the Afghan government said about 140 civilians died, including 92 children.

The Baghdad video shows one of two US Apache helicopter crews falsely claiming that there is shooting, opening fire and then laughing over the dead. The helicopters also attack a van attempting to rescue the wounded. One of them opens fire with armour-piercing shells. One of the crew laughs about the attack after the windscreen is blown out. Behind it were two children who were wounded.

Manning faces two charges under military law for allegedly illegally transferring the Iraq video and copies of documents to his computer and then for passing "national defence information to an unauthorised source". The charge sheet says Manning leaked the material to "bring discredit upon the armed forces".

Although WikiLeaks has not formally acknowledged that Manning is the source of the material it has retained three lawyers in the US to help defend him.

Lamo told Wired magazine that he went to the military and FBI about Manning "because lives were in danger" after the soldier also boasted of leaking thousands of pages of diplomatic cables.

Manning told Lamo that he contacted Assange because what he saw in the Baghdad video disturbed him. "At first glance it was just a bunch of guys getting shot up by a helicopter," Manning wrote. "No big deal … about two dozen more where that came from, right? But something struck me as odd with the van thing, and also the fact it was being stored in a Jag (judge advocate general) officer's directory. So I looked into it."

Manning gave the video to WikiLeaks in February. The site made it public two months later after breaking the encryption.

Lamo said that what disturbed him was a boast by Manning that he had sent 260,000 pages of confidential diplomatic cables to WikiLeaks. "Hillary Clinton, and several thousand diplomats around the world, are going to have a heart attack when they wake up one morning, and find an entire repository of classified foreign policy is available, in searchable format, to the public," Manning is said to have written. Lamo said he met military investigators and the FBI because he feared the leak of diplomatic cables endangered US security. Lamo told Wired: "He was in a war zone and basically trying to vacuum up as much classified information as he could, and just throwing it up into the air."