Organised gangs of international art traffickers were behind the looting of the most important exhibits at Baghdad's national museum, experts believe.
Professor Gibson said gangs had exported artefacts since 1991
Thousands of ancient artefacts dating from the birth of civilisation were stolen and damaged in the wake of the entry of US forces to the city.
Professional smugglers joined opportunist civilians to take advantage of the lawless vacuum, a group of archaeologists has been told.
About 30 of the world's leading experts on Iraqi heritage gathered to discuss the situation at the Paris headquarters of the United Nations' cultural arm Unesco on Thursday.
Professor McGuire Gibson from Chicago University told the group that some thieves obviously knew what they were looking for and where to find it.
"It looks as if part of the looting was a deliberate planned action," he said.
80,000 cuneiform tablets with world's earliest writing
Bronze figure of Akkadian king - 4,500 years old
Silver harp from ancient city of Ur - 4,000 years old
Three-foot carved Sumerian vase - 5,200 years old
Headless statue of Sumerian king Entemena - 4,600 years old
Carved sacred cup - 4,600 years old
"They were able to take keys for vaults and were able to take out important Mesopotamian
materials put in safes."
Some artefacts from the museum have already turned up in Paris and Iran, he said.
"Probably [it was done] by the same sorts of gangs that have been paying for the destruction of sites in Iraq over the last 12
years and the smuggling out of these objects into the international market," Prof Gibson said.
Among the items known to have been lost were 80,000 cuneiform tablets that were some of the world's earliest examples of writing, he said.
Unesco's director general Koichiro Matsuura called for an immediate ban on the international trade in Iraqi antiquities and a "heritage police" to protect Iraq's cultural sites.
US soldiers have started guarding Baghdad's museum
"I believe it is necessary to take emergency measures, such as the setting up... of a nationwide 'heritage police' entrusted with the task of watching over cultural sites and institutions," he said.
Also announced was a special fund for Iraqi cultural heritage, to which Italy, France, Britain, Germany, Egypt and Qatar have already contributed.
"It is always difficult when communities are facing the consequences of an armed conflict to plead the case for the preservation of the cultural heritage," according to Mr Matsuura.
"It is as if we were more interested in stones than in people. But nothing could be further from the truth, of course."
Unesco, which called the loss and destruction a "disaster", will soon send a team to Iraq to find out what is missing and damaged.
The experts were told that some of the country's most prized treasures were hidden in the vaults of the national bank before the war.
But there is still confusion over whether those vaults were looted.
Mr Matsuura said he would press for a UN Security Council resolution to impose a provisional embargo on the acquisition of Iraqi cultural items.
A website with information and pictures of the missing antiquities is also to be set up.
Among those attending Wednesday's meeting were several archaeologists who said they warned the United States Government about the possibility of looting, including Prof Gibson.
US Secretary of State Colin Powell has already pledged to "recover that which has been taken and also participate in restoring that which has been broken".
Iraq is described as a "the cradle of civilisation" and the Baghdad museum contained thousands of irreplaceable artefacts dating back 10,000 years.
The development of abstract counting, the wheel and agriculture were all charted in its exhibitions.
The collections from the Sumerian, Babylonian and Assyrian periods were particularly prized.
As well as the Baghdad museum, the National Archives Centre and a museum in Mosul were looted and the capital's Islamic Library, which housed ancient manuscripts including one of the oldest surviving copies of the Koran, was ravaged by fire.
There are fears that many of the stolen items may be taken out of the country and lost forever.