Governments around the world today braced for the release of millions of potentially embarrassing US diplomatic cables by WikiLeaks as Washington raced to contain the fallout.

The whistle-blower website is expected to put online three million leaked cables covering US dealings and confidential views of countries including Australia, Britain, Canada, Israel, Russia and Turkey.

US diplomats skipped their Thanksgiving holiday weekend and headed to foreign ministries hoping to stave off anger over the cables, which are internal messages that often lack the niceties diplomats voice in public.

"WikiLeaks are an absolutely awful impediment to my business, which is to be able to have discussions in confidence with people. I do not understand the motivation for releasing these documents," said James Jeffrey, the US ambassador to Iraq.

"They will not help, they will simply hurt our ability to do our work here," he told reporters.

The top US military commander, Admiral Mike Mullen, meanwhile urged WikiLeaks to stop its "extremely dangerous" release of documents, according to a transcript of a CNN interview set to air Sunday.

State Department spokesman Philip Crowley also condemned WikiLeaks's plans.

The United States was "gearing up for the worst-case scenario", he added.

Russia's respected Kommersant newspaper said that the documents included US diplomats' conversations with Russian politicians and "unflattering" assessments of some of them.

Russia's Foreign Minister Sergei Lavrov blamed the impending file dump on "little thieves running around the Internet," the Interfax news agency quoted him as saying.

WikiLeaks has not specified the documents' contents or when they would be put online, but Pentagon spokesman Colonel Dave Lapan said officials were expecting a release "late this week or early next week."

The website has said there would be "seven times" as many secret documents as the 400,000 Iraq war logs it published last month.

Turkish media said the planned release includes papers suggesting that Ankara helped Al-Qaeda militants in Iraq and that the United States helped Iraq-based Kurdish rebels fighting against Turkey -- potentially explosive revelations for the two allies.

Foreign Minister Ahmet Davutoglu said Turkey did not know what the documents contained.

"This is speculation," he said on CNN Turk. "But as a principle, tolerating or ignoring any terrorist action that originates in Turkey and targets a neighbouring country, particularly Iraq, is out of the question."

Israel has also been warned of potential embarrassment from the latest release, which could include confidential reports from the US embassy in Tel Aviv, Haaretz newspaper said, citing a senior Israeli official.

The US ambassador in Canada telephoned Foreign Minister Lawrence Cannon about the leak, a ministry spokeswoman said, adding that the Canadian embassy in Washington was "engaging" with the State Department on the matter.