US Defence Secretary Robert Gates has called in the FBI to help investigate who leaked 90,000 classified military documents on the war in Afghanistan.
The documents were posted online on Sunday.
Mr Gates said on Thursday that intelligence sources and methods, as well as military tactics, could "become known to our adversaries".
Visibly angry, he told a news conference the leaks were a massive breach of security that could endanger the lives of American troops, their allies and Afghan informants.
An "aggressive investigation" would determine how the leaks happened, he said.
The Wikileaks website, which posted the documents, said they had been compiled by a variety of US units between 2004 and 2009.
Afghanistan's President Hamid Karzai says the lives of Afghans have been endangered by the leaked documents, which according to the BBC contain the names of hundreds of Afghan informers.