Nov 25: The US was preparing for another WikiLeaks release of classified US documents that could harm relations with countries around the world, the State Department said.

US diplomatic outposts were informing their host countries that the release could come within days, State Department spokesman P.J. Crowley told reporters Wednesday. The release is believed to include sensitive classified documents and State Department cables assessing countries.

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

Typical diplomatic cables contain analysis of situations, as well as records of discussions between US diplomats and foreign officials in what Crowley described as "diplomacy in action".

"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," he 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."

The WikiLeaks release is expected to be the largest leak ever of classified documents. In October, WikiLeaks published nearly 400,000 classified US military documents related to the war in Iraq, prompting Washington to condemn the self-proclaimed whistleblower website for compromising national security and putting lives at risk.

In hinting that a new release was imminent, WikiLeaks said on Twitter Sunday that it will be "7x the size of the Iraq War Logs". In the past, WikiLeaks has provided the documents in advance to The New York Times, The Guardian and Germany's Der Spiegel magazine provided they adhere to a simultaneous publication date.

The State Department has also informed Congress of the latest pending release, Crowley said. He acknowledged the State Department "has known all along" that WikiLeaks obtained the diplomatic cables and was bracing for the publication.

"We wish this would not happen, but we are obviously prepared for the possibility that it will," he said.

The US military arrested Private First Class Bradley Manning and transferred him to the US in July in connection with the leaking to WikiLeaks of 90,000 Afghan documents, and a classified video broadcast earlier this year depicting a 2007 US helicopter attack in Baghdad that killed civilians.

He was charged with illegally accessing and disclosing classified information. US authorities have not said whether Bradley was behind the leaking of the Iraq war logs or the forthcoming State Department documents.

Manning was working as an intelligence analyst in Iraq at the time of his arrest and reportedly had access to classified material. Reports said the military had obtained evidence from his computer showing he had downloaded secret information.

Meanwhile, a Swedish appeals court Wednesday upheld an earlier decision to detain the founder of WikiLeaks, Julian Assange, in connection with accusations of rape. A lower court last week had approved the detention order by prosecutors who suspect the Australian of "rape, sexual molestation and unlawful coercion".

Two women came forward in August with rape allegations against Assange while he was in Sweden. Assange has rejected the accusations.