Iraq war costs $US 3 trillion

Joseph Stiglitz's book The Three Trillion Dollar War: The True Cost of the Iraq Conflict, concluded that US military operations in Iraq already have exceeded the cost of the 12-year war in Vietnam and is more than double the cost of the Korean War.

"The only war in our history which cost more was the Second World War, when 16.3 million US troops fought in a campaign lasting four years, at a total cost (in 2007, inflation-adjusted dollars) of about $US5 trillion ($A5.4 trillion)," he wrote in the work co-authored with Harvard professor Linda Bilmes.

"With virtually the entire armed forces committed to fighting the Germans and Japanese, the cost per troop (in today's dollars) was less than $US100,000 ($A108,000) in 2007 dollars.

"By contrast, the Iraq war is costing upward of $US400,000 ($A432,000) per troop," Mr Stiglitz writes.

The Pentagon took exception to the figures - and to the premise that there have been undisclosed costs involved in financing the war.

"It seems like an exaggerated number to us," said Pentagon spokesman Geoff Morrell.

"The Pentagon has been extraordinarily transparent about what we know of the cost of the war."

Mr Stiglitz won the 2001 Nobel Prize in economics and was formerly chief economist at the World Bank.

The book's co-author Linda Bilmes is a professor of public finance at Harvard University.

The book is published as the fifth year of the US-led conflict comes to a close.