The US has fewer troops in Iraq than Vietnam, but pays them more

The report put costs in Iraq at $500m (£278m) a month more than in Vietnam, adjusted for inflation.

This makes Iraq the most expensive US war in the past 60 years, they say.

But an analyst from the conservative American Enterprise Institute (AEI) said the cost was small in the context of the whole US economy.

The report by the Institute for Policy Studies (IPS) and Foreign Policy in Focus (FPIF), called The Iraq Quagmire, calculates the cost of current military operations in Iraq at $5.6bn (£3.1bn) every month.

By comparison, the eight-year campaign in Vietnam cost on average $5.1bn (£2.8bn) a month.

'Poor preparation'

The IPS and FPIF say this is partly down to differences in the way modern war is waged.

Although there are fewer troops in Iraq than Vietnam, they are paid more and weapons are more expensive, the report says.

Weapons have become more expensive since the Vietnam War

"Broken down per person in the US, the cost so far is $277 per person, making the Iraq War the most expensive military effort in the past 60 years," it concludes.

Co-author Erik Leaver told the BBC costs in Iraq had spiralled since 2003 because the US had not been well-prepared.

"We have deployed now roughly one million troops in Afghanistan and Iraq, and the numbers just keep going up and up," he said.

"We are going to continue to see costs not only from the fighting now but also from the health care of these soldiers and veterans when they come home."

'Drop in the bucket'

However Thomas Donnelly, a defence expert with the AEI, believes the eventual result of the Iraq war is more important than its cost in dollars.

"The more valued criticism is whether the Bush administration is winning the war and prosecuting it in a successful way," he said.

"So what price victory? I would say that $5bn a month is certainly something I would be willing to pay."

Mr Donnelly said the relative cost of operations in Iraq, at 2% of America's annual GDP, was less than either the Vietnam conflict at 12% or World War II at 40%.

"Although the costs of war have grown... the American economy is exponentially larger than it was in the Vietnam War years," Mr Donnelly said.

"When it [the Iraq war] is compared to the overall size of the American economy, it's really a drop in the bucket, certainly by historical standards."