"We are all bracing for what may be coming and condemn WikiLeaks for the release of classified material," State Department spokesman Philip Crowley said.
"It will place lives and interests at risk. It is irresponsible."
The United States was "gearing up for the worst-case scenario," he added, and Washington had been briefing governments about the leak.
WikiLeaks is expected to put online three million leaked cables covering US dealings and its confidential views of other countries.
Many fear it will embarrass the United States and its allies and reveal sensitive details about US relations with other countries.
"Across the State Department, senior officials are reaching out to countries and warning them about a possible release of documents," Crowley said.
Secretary of State Hillary Clinton had contacted leaders in Germany, Saudi Arabia, the United Arab Emirates, Britain, France and Afghanistan regarding WikiLeaks, he added.
WikiLeaks has not specified the documents' contents or when they will be put online.
But the website has said there would be "seven times" as many secret documents as the 400,000 Iraq war logs it published last month.
Senior US officials have already warned about the dangers of leaking the documents.
The top military commander in the United States said the website must stop its "extremely dangerous" release of sensitive documents, according to a CNN transcript released Friday.
"I would hope that those who are responsible for this would, at some point in time, think about the responsibility that they have for lives that they're exposing... and stop leaking this information," Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, told CNN's "Fareed Zakaria GPS."
"It continues to be extremely dangerous," Mullen said.
