"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."
Bradley Manning, a US army intelligence analyst, was charged earlier this month in connection with the leak of a classified video, showing a 2007 helicopter attackthat killed a dozen civilians in Baghdad, to Wikileaks.
The Pentagon said in June that it was investigating allegations that Manning had handed over classified video and 260,000 secret diplomatic cables to the website.
It was not immediately clear if he was being investigated about the leak of the files on the war in Afghanistan and the Pentagon has declined to name any suspects.
The unverified files suggest that Pakistan's intelligence agency has been holding strategy sessionswith Taliban leaders to aid their efforts in Afghanistan.
The documents also include descriptions of a covert US special operations unit formed to target high-level al-Qaeda and Taliban figures, incidents that caused civilian casualties and a host of other operational reports.
The Pentagon said its review of the documents made public 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 the significance of what had emerged so far, saying that they appeared to be low-level assessments that largely confirm the military's publicly stated concerns about the Afghan war.
"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.
The Pentagon said it was also looking at possible damage to the war effort on the ground in Afghanistan.
Michael Hayden, a former CIA director, said the leak was a gift to the enemies of the United States.
"If I had gotten this trove on the Taliban or al-Qaeda, I would have called it priceless," he said.
He predicted that the Taliban would take anything that described a US attack and the intelligence behind it "and figure out who was in the room when that particular operation, say in 2008, was planned, and in whose home".
Then the fighters would likely punish the traitor who had worked with the Americans, Hayden said.
Jane Harman, a Democratic congresswoman, said the White House had indicated the disclosures compromised a number of Afghan sources.
PJ Crowley tells Al Jazeera most of the leaked documents on Wikileaks are old news
"Someone inadvertently or on purpose gave the Taliban its new enemies list," she said.
The leak of classified documents could create deeper doubts about the war at home, cause new friction with Pakistan over allegations about its spy agency and raise questions around the world about Washington's ability to protect military secrets.
The White House called the leak, which is one of the biggest in US military history, "alarming".
But Robert Gibbs, the White House spokesman, emphasised that the documents covered the period before Barack Obama, the president, ordered a major increase in US troops fighting in Afghanistan, and the administration denied they would cause any policy shift in the fight against the Taliban.