Manning's alleged action of supplying classified video and diplomatic communications to Wikileaks was first reported by Wired.com, the website of technology magazine Wired.
Bryan Whitman, a Pentagon spokesman, said investigators were probing allegations that Manning supplied classified video and 260,000 secret diplomatic cables to Wikileaks.
"I think that's why the Criminal Investigative Division is taking a very scrupulous look at this," Whitman said in Washington.
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Wired said Manning, from Maryland, was arrested nearly two weeks ago by the US Army's Criminal Investigation Division at Forward Operating Base Hammer, 64km east of Baghdad.
Philip Crowley, a US state department spokesman, said the department would take the leak of classified documents "seriously".
"It has particular impact in terms of revealing what we call sources and methods, compromising our ability to provide government leaders with the kind of analysis that they need to make informed decisions," Crowley said.
Wikileaks, a website that publishes anonymously sourced documents, released what it called previously unseen footage of the Apache helicopter attack in Baghdad in April.
At the time Wikileaks said only that it had obtained the video "from a number of military whistleblowers" but did not provide any further information on how it got hold of it.
"I wouldn't have done this if lives weren't in danger. He [Manning] 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"
Adrian Lamo, former hacker
In a Twitter feed Wikileaks said "allegations in Wired that we have been sent 260,000 classified US embassy cables are, as far as we can tell, incorrect".
It said that "if" Manning was the "whistleblower then, without doubt, he's a national hero".
Manning reportedly said he had leaked other material to Wikileaks, including a separate video of a 2009 air strike in Afghanistan, a classified army document evaluating Wikileaks as a security threat and classified US diplomatic cables, according to Wired.
Wired said Manning had been in touch with former hacker Adrian Lamo, who contacted army investigators and FBI agents after being told of the leaks.
"I wouldn't have done this if lives weren't in danger," Lamo told Wired about turning Manning in to the authorities.
"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."