The London-based Guardian newspaper and other outlets obtained the documents from WikiLeaks in advance of the self-described whistleblower website's plan to publish them on Saturday. The papers number more than 390,000 classified US military documents and their publication has brought strong protests from the Pentagon.

The Guardian says the documents show US authorities failed to investigate allegations of crimes committed by Iraqi police and soldiers. US authorities said any known crimes were reported to the appropriate Iraqi officials responsible for governing the country.

WikiLeaks provided several news outlets with the documents in advance, including The New York Times and the German news magazine Der Spiegel. The documents, covering a five-year period beginning in 2004, mostly consist of field reports from the battlefront.

The New York Times reported that the documents showed a US Apache helicopter in 2007 gunned down and killed two insurgents who had been firing mortars and then began signalling a desire to surrender. But according to the document, a military lawyer said they cannot surrender to aircraft, and are still valid targets."

The Guardian said the documents showed more than 15,000 civilians had died than were previously reported, and that although US and British officers said there was no official body count for civilians they were indeed keeping one that showed there were 66,081 non- combatant deaths.

The New York Times said the field reports show that most Iraqi civilians "by far" were killed by other Iraqis, but also said American soldiers were responsible for killing some Iraqis at checkpoints.

WikiLeaks in July angered the Pentagon by leaking more than 90,000 documents related to the war in Afghanistan. Although the documents contained little new information, the Pentagon worried they would reveal sources, methods and tactics that could give insight on how US forces operate and make them vulnerable to attacks. They also said it could put the lives of individuals collaborating with the United States at risk.

Anticipating the release, US Secretary of State Hillary Rodham Clinton earlier condemned WikiLeaks for making the classified material public. The leaking of the documents should be condemned in the "most clear terms" because it could place US soldiers and other personnel in danger and threatens US national security as well as that of "those with whom we are working."

The US military has arrested Private First Class Bradley Manning and transferred him to the United States in July in connection to leaking to WikiLeaks the Afghan documents and a video published earlier this year depicting a 2007 US helicopter attack in Baghdad that killed some civilians. They have not said whether Bradley could have been involved in the latest release.

Manning was working as an intelligence analysts 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.

Like with the Afghan documents, the Pentagon downplayed the significance of the documents, saying most of them contain information that has already been publicly reported and are "old news."

"There probably won't be any big surprises," Major Chris Perrine, a Pentagon spokesman, told German Press Agency dpa hours before the documents were reported. "It's just old news."