The Obama administration has alerted Congress and foreign governments of the expected release of more sensitive documents on the WikiLeaks website.
They fear the release of US diplomatic files could damage relations with friends and allies across the globe.
"These revelations are harmful to the US and our interests," said State Department spokesman PJ Crowley. "They are going to create tension in relationships between our diplomats and our friends around the world."
Mr Crowley said the release of confidential communications about foreign governments would probably erode trust in the US as a diplomatic partner and could cause embarrassment if the files should 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," he said.
US diplomatic outposts around the world had begun notifying other governments that WikiLeaks may release these documents in the coming days, Mr Crowley said.
A major focus of the documents is Europe, but the cables are likely to touch on relations with many important countries in Asia and elsewhere, another official said. The release is expected to happen this weekend, although WikiLeaks has not been specific about the timing.
In two previous releases of leaked secret US government documents, in July and October, WikiLeaks provided them in advance to The New York Times, the Guardian newspaper 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.
Diplomatic cables are internal documents that would include a range of secret communications between US 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 US and foreign government officials, to the explosive, in the case of candid assessments of foreign officials or the exposure of pressure tactics used by US 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 programmes could be exposed. These might include details about surveillance at US diplomatic compounds abroad or revelations about highly secret intelligence sources or practices.