Afghan President Hamid Karzai says the release of secret U.S. military documents have endangered the lives of Afghan citizens who have co-operated with NATO forces. ((Musadeq Sadeq/Associated Press))

Afghan President Hamid Karzai condemned the leak of thousands of secret U.S. defence documents relating to the war in Afghanistan, saying it has put the lives of Afghan informants working with NATO and U.S. forces at risk.

"This, indeed, is very irresponsible and shocking," Karzai told reporters at a press conference in the Afghan capital Kabul Thursday.

"Their lives will be in danger now. This is a very serious issue."

According to the New York Times, some of the roughly 75,000 U.S. military field reports WikiLeaks has released on its website to date contain names or other identifying details about Afghans who have been co-operating with NATO forces, informing on the Taliban or who have been pegged as potential defectors or informants.

The sampling of documents published by the Times and two other media — the Guardian and Der Spiegel — on Monday had names and identifying information redacted.

WikiLeaks said it held back 15,000 of the total 91,000 documents it received in order to remove names and personal information, but the Times said it found documents that did contain such details among the material already posted on the group's site.

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Julian Assange, founder of WikiLeaks, a website devoted to publishing leaked government and other sensitive documents, said if names have been revealed, it was a mistake and something his organization would take seriously. 

But he also insisted that if people have been put at risk, it's "because of a misclassification by the U.S. military itself."

The Pentagon is reviewing the documents released by WikiLeaks to see whether any Afghans are at risk of reprisals.

On Thursday, U.S. Defence Secretary Robert Gates said a criminal investigation into the leak could go beyond the military, and he did not rule out that Assange could be a target.

"The investigation should go wherever it needs to go," Gates said.

He would not be more specific, waving off questions about whether Assange or media outlets that used the WikiLeaks material could be subjects of the criminal probe.

But he noted that he has asked the FBI to help in the investigation "to ensure that it can go wherever it needs to go."

With files from The Associated Press