Military documents laid bare in the biggest leak of secret information in US history suggest that far more Iraqis died than previously acknowledged during the years of sectarian bloodletting and criminal violence unleashed by the 2003 US-led invasion.

The accounts of civilian deaths among nearly 400,000 purported Iraq war logs released on Friday by the WikiLeaks website include deaths unknown or unreported before now – as many as 15,000 by the count of one independent research group.

The field reports from US forces and intelligence officers also indicate US forces often failed to follow up on credible evidence that Iraqi forces mistreated, tortured and killed their captives as they battled a violent insurgency.

The war logs were made public in defiance of Pentagon insistence that the action puts the lives of US troops and their military partners at risk.

Although the documents appear to be authentic, their origin could not be independently confirmed, and WikiLeaks declined to offer any details about them.

The 391,831 documents date from the start of 2004 to 1 January, 2010, providing a ground-level view of the war written mostly by low-ranking officers in the field. The dry reports, full of military jargon and acronyms, were meant to catalogue "significant actions" over six years of heavy US and allied military presence in Iraq.

The Pentagon has previously declined to confirm the authenticity of
WikiLeaks-released records, but it has employed more than 100 analysts to review what was previously released and has never indicated that any past WikiLeaks releases were inaccurate.

Casualty figures in the US-led war in Iraq have been hotly disputed. Critics on each side accuse the other of manipulating the death toll to sway opinion.

Iraq Body Count, a private British-based group, said it had analysed the information and found 15,000 previously unreported deaths, which would raise its total from as many as 107,369 civilians to more than 122,000 civilians.

The Iraqi government has issued a tally of at least 85,694 deaths of civilians and security officials between January 2004 and 31 October, 2008.

That US body count, reported this month, tallied deaths of almost 77,000 Iraqis between January 2004 and August 2008 – the darkest chapter of Iraq's sectarian warfare and the US troop surge to quell it. The new data was posted on the US Central Command website without explanation.