The latest tranche of documents, the third since WikiLeaks published 77,000 classified US files on the Afghan conflict in July, have spurred Washington to warn both Turkey and Israel of the embarrassment they could cause, and American diplomats have also briefed officials in London, Oslo and Copenhagen.
"We are worried about additional documents coming out," US ambassador to Baghdad James Jeffrey told reporters at an embassy briefing.
"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.
"They will not help, they will simply hurt our ability to do our work here."
Asked what Iraqi officials had said to him about the release of the documents, Jeffrey replied: "They clearly are very unhappy... anybody who has confidential discussions, who finds these confidential discussions find their way into the press, is going to be very unhappy and very upset."
State Department spokesman Philip Crowley told AFP on Wednesday that the United States was "gearing up for the worst-case scenario, that leaked cables will touch on a wide range of issues and countries."
"We are prepared if this upcoming tranche of documents includes State Department cables. We are in touch with our posts around the world. They have begun the process of informing governments that a release of documents is possible in the near future," Crowley said.
He added: "These revelations... are going to create tensions on our relationships between our diplomats and our friends around the world."
WikiLeaks has not specified what the tranche of documents pertains to or when it would be released, but Pentagon spokesman Colonel Dave Lapan said US officials were expecting a possible release of documents "late this week or early next week."
The website has so far only said there would be "seven times" as many secret documents as the 400,000 it posted in Iraq war logs published last month.
Among the countries to have been alerted so far about the release of the documents are Britain, Denmark, Israel, Norway and Turkey, officials and reports said.
Washington contacted authorities in Ankara to give "us information on the issue, just as other countries have been informed," a senior Turkish diplomat told AFP, speaking on condition of anonymity.
According to Turkish media reports, the planned release includes papers suggesting that Turkey helped Al-Qaeda militants in Iraq, and that the United States helped Iraq-based separatist Kurdish rebels fighting against Turkey.
Israel has also been warned of potential embarrassment from the release, which could include confidential reports from the US embassy in Tel Aviv, Haaretz newspaper said on Thursday, citing a senior Israeli official.
"The Americans said they view the leak very seriously," the official told the paper, on condition of anonymity.
And respected Russian business daily Kommersant reported on Friday that the files could harm Moscow's relationship with Washington, saying the cables contain general assessments of the political situation in Russia and "unflattering characteristics" of Russian leaders.
Officials in London, Stockholm and Copenhagen were also all either briefed by US diplomats or received contacts from local American missions about the impending release, officials in each country said.
WikiLeaks argues the release of the documents -- US soldier-authored incident reports from 2004 to 2009 -- has shed light on the wars in Afghanistan and Iraq, including allegations of torture by Iraqi forces and reports that suggested 15,000 additional civilian deaths in Iraq.
Its announcement on Monday came just days after Sweden issued an international arrest warrant for the website's head, Julian Assange of Australia, wanted for questioning in connection with allegations of rape and sexual molestation.
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