Officials said the documents may contain everything from accounts of compromising conversations with political dissidents and friendly politicians to disclosures of activities that could result in the expulsion of U.S. diplomats from foreign postings.
U.S. diplomatic outposts around the world have begun notifying other governments that WikiLeaks may release these documents in the next few days.
"These revelations are harmful to the United States and our interests," State Department spokesman P.J. Crowley said. "They are going to create tension in relationships between our diplomats and our friends around the world."
Crowley said the public airing of what were supposed to be private communications will likely erode trust in the U.S. as a diplomatic partner. And he said they could cause embarrassment if the files include derogatory or critical comments about friendly foreign leaders.
"When this confidence is betrayed and ends up on the front pages of newspapers or lead stories on television or radio, it has an impact," Crowley said.
The release is expected this weekend, although WikiLeaks has not been specific about the timing.
Officials speaking on condition of anonymity to discuss sensitive internal matters said the administration believes that the diplomatic fallout could be substantial. Many of the cables are believed to date from the start of the Obama administration, meaning that the White House won't be able to distance itself from any disclosures.
U.S. officials are concerned that some of the leaked cables could include details of conversations in which senior foreign politicians offer candid appraisals of their governments. Those assessments could prove embarassing, not only to the United States but to the politicians and governments concerned.
Some of the documents may provide specifics of meetings U.S. diplomats have had with opposition leaders, dissidents or human rights activists in various countries that could expose them to retaliation, particularly in authoritarian nations, officials said.
In more extreme cases, officials fear the cables could divulge embassy activities or analyses that might result in American diplomats being expelled from foreign countries.
A key focus of the documents is Europe, but the cables are likely to touch on relations with many key countries in Asia and elsewhere, another official said, speaking anonymously in order to discuss internal deliberations.
Crowley said the State Department "has known all along" that WikiLeaks possesses classified State Department documents. He said it was not possible, however, to predict with precision the impact of their release because the State Department does not know which files will be released.
"We wish this would not happen, but we are obviously prepared for the possibility that it will," he said.
In two previous releases of leaked secret U.S. government documents, in July and October, WikiLeaks provided them in advance to The New York Times, the Guardian newspaper in London and the German magazine Der Spiegel on condition that they publish their stories simultaneously.
The first leak contained thousands of military field reports on the war in Afghanistan; the second was a similar but larger file on the Iraq war.
No one has been charged with providing the documents to WikiLeaks, but a person of interest in the Pentagon's investigation is Army Pfc. Bradley Manning, who was an intelligence analyst in Iraq when he was arrested by U.S. authorities in early June and jailed in Kuwait. On July 29 he was transferred to a brig at Quantico Marine Corps Base in Virginia.
Diplomatic cables are internal documents that would include a range of secret communications between U.S. diplomatic outposts and State Department headquarters in Washington.
The revelations they contain are likely to range from the mundane, in the case of routine reporting on meetings between U.S. and foreign government officials, to the explosive, in the case of candid assessments of foreign officials or the exposure of pressure tactics used by U.S. diplomats.
One concern, for example, is that the documents may reveal the kinds of pressure the Obama administration has put on various countries to accept the transfer of Guantanamo Bay detainees who have been cleared for release but are unwelcome in their home countries.
State Department officials said privately there was concern, too, that details about certain sensitive programs could be exposed. These might include details about surveillance at U.S. diplomatic compounds abroad or revelations about highly secret intelligence sources or practices.
A Pentagon spokesman, Marine Col. David Lapan, said the Pentagon also has notified congressional committees of an expected WikiLeaks release. He said the files are believed to be State Department documents, but they could contain information about military tactics or reveal the identities of sources.
A statement on WikiLeaks Twitter site Wednesday said "the Pentagon is hyperventilating again over fears of being held to account."
The group bills itself as a website devoted to reforming governments worldwide by exposing their secrets, and its motto on its Twitter site is "We open governments."
Another recent posting said: "The coming months will see a new world, where global history is redefined."