US defense officials said the person behind the release of some 91,000 classified documents appeared to have "secret" clearance and access to sensitive documents on the Afghan war.
More leaks were possible, officials acknowledged.
"We will do what is necessary to try to determine who is responsible for the leaking of this information," Pentagon press secretary Geoff Morrell said.
"Until we know who's responsible, you have to hold out the possibility that there could be more information that has yet to be disclosed. And that's obviously a concern."
The Pentagon said its review of the documents being made public by the organization WikiLeaks would take "days if not weeks" and that it was too soon to assess any damage to national security.
Still, US military officials played down any revelations within the documents revealed so far, saying they appeared to be low-level assessments that largely confirm the military's publicly stated concerns about the Afghan war.
The military's warnings of potential mission failure last year helped lead to President Barack Obama's decision in December to deploy 30,000 more troops.
"The scale of (the leak), the scope of it, is clearly alarming. I don't think the content of it is very illuminating," Morrell said.
Among the documents were reports that U.S. officials in Afghanistan strongly suspected Pakistan was secretly supporting Taliban insurgents while taking massive amounts of American aid. The documents could fuel growing doubts in Congress about Obama's war strategy as the U.S. death toll rises.
The Pentagon has declined to name any suspects over the leak but also has refused to rule out potential involvement of an Army specialist already awaiting trial on charges of leaking information related to the Iraq war to WikiLeaks.
Army Specialist Bradley Manning was charged earlier this month in connection with the leak of a classified video showing a 2007 helicopter attack that killed a dozen people in Iraq, including two Reuters journalists. He is also accused of downloading State Department cables to his personal computer.
Asked whether WikiLeaks also might come under scrutiny, Pentagon officials have said that historically the leakers have been the ones targeted for criminal prosecution -- not those who merely publish the information.
"I don't know what's going to happen here. This is a whole new world that we are entering into where an organization without any editorial judgment, beholden to nobody, is soliciting classified information from people all over the world and then publishing it," Morrell said.
"I don't know. I'm not a lawyer but people are going to have to make judgments about whether there are legal ramifications for soliciting a criminal act."