US Defence Secretary Robert Gates has called in the FBI to help with the inquiry into the leaking of more than 90,000 classified military records.
The documents, published online on Sunday, give details of the Afghan war.
Mr Gates said they were potentially dangerous for US troops and allies, and an "aggressive investigation" would determine how the leaks occurred.
The Wikileaks website, which posted the documents, said they had been compiled by a variety of US units in 2004-09.
"The battlefield consequences of the release of these documents are potentially severe and dangerous for our troops, our allies and Afghan partners, and may well damage our relationships and reputation in that key part of the world," Mr Gates told reporters on Thursday.
He said intelligence sources and methods, as well as military tactics could "become known to our adversaries".
Mr Gates added that he had called FBI director Robert Mueller on Wednesday and "asked for the FBI's assistance in our investigation as a partner".
"It is important that we have all the resources we need to investigate and assess this breach of national security," he said.
Meanwhile, the Pentagon is looking into a possible connection between soldier Bradley Manning and the new document leaks, according to a statement issued by the US Department of Defense.
Pfc Manning, an army intelligence analyst, was charged with leaking a video of a US helicopter attack in Iraq to Wikileaks last month.
The probe into the document leaks is an extension of the existing open investigation on Pfc Manning, Marine Corps Col Dave Lapan said in a statement.
Wikileaks has said it tried hard to ensure that the release of the material "does not put innocents at harm", and held back about 15,000 reports.
One of the leaked reports, from 2007, suggests how civilian casualties could be underplayed.
This initial report makes no mention of civilian casualties as a marine convoy near Jalalabad opened fire while tearing back to base after being rammed by an explosives-filled van.
An update to the report states eight Afghan civilians were killed and 34 wounded. A later Afghan Human Rights Commission report into the incident found that 19 civilians had been killed as the marines drove down the highway firing their weapons.
Another leaked report claims members of Pakistan's Inter-Services Intelligence (ISI) agency had backed the Taliban in the war in Afghanistan - an accusation strongly denied by Islamabad.