By James Menendez Middle East reporter, BBC World service

The looting of the National Museum in Baghdad two years ago caused an international outcry.

Satellite photos show the extent of damage caused by looting

In the chaos that engulfed the city at the end of the war, thousands of pieces were either stolen or damaged.

The US marines who first captured Baghdad were accused of not doing enough to protect the museum's priceless collection of Mesopotamian art.

Others suspected an inside job, with professional smugglers, members of Saddam Hussein's inner circle and international dealers all in the frame.

The precise circumstances of what happened are still not clear, although the loss to Iraq's cultural heritage is not disputed.

But now, one of the world's leading experts on the country's antiquities says there is evidence that archaeological treasures are being systematically plundered.

We know there are people sitting in Saudi Arabia and in Jordan, asking for specific material from specific sites Dr Donny George Baghdad museum

"What's going on here is worse than what happened with the Baghdad museum," says Professor Elizabeth Stone of Stonybrook University in New York.

"What happened at the museum shouldn't have happened. But in terms of what was taken, we knew where it came from. We have photographs. What's coming out of here, we haven't the faintest idea what it is."

Professor Stone has been studying new satellite images which show hundreds of neatly-arranged holes where sites have been dug up.

"We can tell the difference between the areas they're really targeting and the areas they're probing," she says. "We can really make a distinction between different types of looting."

Birthplace of civilisation

Since the end of the war, a special force has been set up to guard these areas. But it is struggling to cope. About 1,000 officers have to protect nearly 10,000 sites.

Guards were deployed at the Baghdad museum after looting

"Archaeological sites are being destroyed in order to find these objects," says Dr John Curtis, head of the Ancient Near East department at the British Museum in London.

"In the process of that looting, very important archaeological evidence gets lost. And it's this evidence that can tell us a great deal about the civilisation."

Ancient Mesopotamia - modern Iraq - is often called the cradle of civilisation.

It is a description richly deserved, says Dr Curtis, as Mesopotamia is the place where writing, medicine, mathematics and astronomy all began.

'Unscrupulous collectors'

The historical importance of the region is what is driving efforts to protect Iraq's museums and sites, but it is also what is driving the thriving market in stolen antiquities. Even a small clay tablet can fetch several hundred dollars.

Somewhere there must be warehouses bulging at the seams because this stuff isn't showing up on the market University professor Elizabeth Stone

Dr Donny George was working at the Baghdad museum when it was ransacked. He is now in charge of the remainder of the collection which, two years on, is still under lock and key.

He says unscrupulous private collectors are the real culprits for what is happening to Iraq's archaeological sites.

"There's a definite connection between the looters and the collectors outside the country," he says.

"We know there are people sitting in Saudi Arabia and in Jordan, asking for specific material from specific sites."

'Underground' trade

But tracking down these pieces is not easy. Some national police forces, as well as Interpol, keep a database of the most important missing artefacts and in many countries it is now illegal to trade in antiquities from Iraq.

That has had some success. But Professor Stone believes it has also driven the trade underground and made it more difficult to detect.

"Somewhere there must be warehouses that are bulging at the seams because this stuff isn't showing up on the market," she says.

"The people who are storing it are perhaps long-term family firms of antiquities dealers. They may be assuming that if it's not this generation then it's the next generation that's going to reap the profits."

The United Nations cultural organisation, Unesco, is reviewing its work on stopping the trade at a conference this week. But it has its work cut out.

Resources are limited. The security situation in Iraq shows little sign of improving.