The US has warned India and other key governments across the world about a new potentially embarrassing release of classified documents by the whistle-blowing website WikiLeaks which may harm the American interests and create tension in its ties with its "friends".

"We have reached out to India to warn them about a possible release of documents," State Department spokesman P.J. Crowley told PTI. "We do not know precisely what WikiLeaks has or what it plans to do. We have made our position clear. These documents should not be released," Mr Crowley said, ahead of the expected release by the website of millions of sensitive diplomatic cables.

It is not known yet what is contained in these documents about India-related issues. The WikiLeaks has said there would be "seven times" as many secret documents as the 400,000 Iraq war logs it published last month.

On his Twitter account, Crowley said the State Department officials have also contacted leaders in Germany, Saudi Arabia, the UAE, Britain, France and Afghanistan.

Secretary of state Hillary Clinton too reached out to Chinese foreign minister Yang Jiechi, as the WikiLeaks, the whistle-blower website, is expected to release some three million classified US cables involving some of its key allies including Australia, Britain, Israel, Russia, Turkey and India.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, urged WikiLeaks to stop "dangerous" leaks. "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 Mullen, chairman of the Joint Chiefs of Staff, told CNN.

"It continues to be extremely dangerous," he said. "We are very mindful of the announcement that WikiLeaks made earlier this week, that there is a release of documents pending at some point in the future."

If the past is prologue, that would mean that certain news organisations may well already be in possession of specific documents, Crowley told reporters early this week.

"So we continue to work through, as we have throughout this process, evaluating both the material that we think was previously leaked from government sources to WikiLeaks and we continue to make clear that this is harmful to our national security. It does put lives at risk. It does put national interests at risk," he said.

"Inherent in this day-to-day action is trust that we can convey our perspective to other governments in confidence and that they can convey their perspective on events to us," Crowley said.

"And when this confidence is betrayed and ends up on the front pages of newspapers or lead stories on television and radio it has an impact."

These revelations are "harmful" to the United States and its interests, he said. "They are going to create tension in our relationships between our diplomats and our friends around the world."

Without getting into any discussion of any specific cables, Crowley said the kinds of cables that US missions across the world send to Washington are classified.

"They involve discussions that we've had with government officials, with private citizens. They contain analysis. They contain a record of the day-to-day diplomatic activity that our personnel undertake," he said.

"This back and forth between government, the government of the United States and governments around the world, it is diplomacy in action. It is part of the system through which we collaborate and cooperate with other countries," he said.