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WASHINGTON (AFP) - The United States on Monday said it was carrying out a criminal investigation into WikiLeaks and would pursue the whistle-blower website's chief if he were found to have broken the law.

"We have an active, ongoing criminal investigation with regard to this matter," Attorney General Eric Holder told a press conference the day after the website began releasing some 250,000 confidential State Department memos.

"We are not in the position, as yet, to announce the result of that investigation," he said, adding that the justice and defense departments were both probing the website.

"This is not saber-rattling," Holder said when pressed by reporters over what action the United States could take against WikiLeaks founder Julian Assange, 39, a shadowy Australian hacker believed to be based in Europe.

"To the extent that we can find anybody who was involved in the breaking of American law... they will be held responsible," Holder said.

"To the extent there are gaps in our laws, we will move to close those gaps. It is not the case that anybody at this point, because of their citizenship or their residency, is not a target or the subject of an investigation."

The US diplomatic cables, most from the last three years, reveal a wide array of shocking diplomatic episodes, as well as candid assessments of world leaders and controversial closed-door remarks.

The White House has strongly condemned the release of the documents, saying it places lives at risk and threatens national security.

US Republican congressman Peter King, the ranking member of the House of Representatives' Homeland Security Committee, has urged the attorney general to prosecute Assange for espionage.

He has also said the State Department should designate WikiLeaks a "Foreign Terrorist Organization," saying it "posed a clear and present danger to the national security of the United States," in a statement on Sunday.

WikiLeaks has defended the release of the documents, saying they serve the public by exposing "the contradictions between the US's public persona and what it says behind closed doors."

It has also defended earlier document dumps of nearly 500,000 US military incident reports from 2004 to 2009, saying they shed light on the wars in Afghanistan and Iraq.

Assange has denied that any individuals have been harmed because of the disclosures.

US officials have not confirmed the source of the leaks, but suspicion has fallen on Bradley Manning, a former army intelligence agent arrested after the release of a video showing air strikes that killed reporters in Iraq.