BAGHDAD (Reuters) - A U.S. soldier has been arrested in connection with the release of a classified video showing a 2007 helicopter attack that killed a dozen people in Baghdad, including two Reuters news staff, the U.S. military said on Monday.

Army Specialist Bradley Manning, 22, of Potomac, Maryland, who was deployed to Baghdad, is being held in pretrial confinement in Kuwait, "for allegedly releasing classified information," according to a U.S. military statement.

The statement did not provide details of the information in question but in an emailed response to a query, a U.S. official confirmed that the case involved a U.S. military videotape made public in April by WikiLeaks, a group that promotes the leaking of information to fight government and corporate corruption.

The gunsight video shows an attack by a U.S. Apache helicopter on a group of men in a square in a Baghdad neighborhood. The group included Reuters photographer Namir Noor-Eldeen, 22, and his assistant and driver, Saeed Chmagh, 40.

Wired magazine, which initially reported the arrest on its website on Sunday evening, said Manning, an intelligence analyst, came under suspicion after he told a former hacker during an email exchange that he had leaked the video. Wired did not disclose its sources but quoted from what it said Manning had written.

The magazine said Manning also claimed to have leaked other classified information, including video of a 2009 bombing in Afghanistan that killed dozens of civilians and 260,000 classified U.S. diplomatic cables.

Wired reported that WikiLeaks had previously acknowledged it was in possession of the bombing video. It said only one U.S. diplomatic cable had been posted by WikiLeaks.

There was no immediate comment from WikiLeaks, but a person involved with the group said WikiLeaks did not know if Manning used its services because all its sources are anonymous.

"Our legal advisers have started working on the case, and are verifying how much this military investigation is violating the rights of our sources and us," the person involved with WikiLeaks said in an email.

The Pentagon said investigators were taking a very careful look at what classified information might have been divulged by Manning, who was deployed with the 2nd Brigade 10th Mountain Division.

NATIONAL SECURITY AFFECTED

"The Department of Defense takes the management of classified information very seriously because it affects our national security, the lives of our soldiers, and our operations abroad," the U.S. military statement said.

"The results of the investigation will be released upon completion of the investigation," it said.

The U.S. State Department said it was cooperating with the Defense Department in the investigation and took the release of any classified cables seriously. It said the cables would have included analysis that had been shared with the military.

"It has particular impact in terms of potentially revealing what we call 'sources and methods,' you know, compromising our ability to ... provide government leaders with the kind of analysis that they need to make informed decisions," State Department spokesman P.J. Crowley told reporters in Washington.

He said it was difficult to assess the effect of the security breach. "What the impact of this will be we'll evaluate over time," Crowley said.

The video of the Iraq helicopter attack released in April included an audio track of conversation between the fliers and showed an aerial view of men moving through the square. The helicopter opened fire, killing several people and wounding others.

A military spokesman said the helicopter crew mistook a camera for a rocket-propelled grenade launcher.

Minutes later, a van approached and began trying to assist the wounded. The fliers apparently became concerned that the vehicle was occupied by militants and fired on it.

WikiLeaks said it obtained the video from military whistleblowers and had been able to view and investigate it after breaking an encryption code.

Some international law and human rights experts say the helicopter crew may have acted illegally.

U.S. Defense Secretary Robert Gates criticised WikiLeaks for releasing the video without providing any context.

The U.S. military has said an investigation shortly after the incident found that the U.S. forces were unaware of the presence of news staff and thought they were engaging armed insurgents.

Reuters editor-in-chief David Schlesinger in April called for a new investigation of the incident.

Military officials said after the video's release that they had no plans to reopen the investigation.

(Reporting by Jim Loney in Baghdad and David Alexander and Arshad Mohammed in Washington; editing by Myra MacDonald and Mohammad Zargham)