Highly organised gangs of looters were able to escape with some of Iraq's most historical treasures because they had keys to the vaults and safes of the country's museums.

Unesco, the UN cultural agency, gathered 30 art experts and cultural historians in Paris yesterday. They said that the nature of the pillaging appeared to have been highly professional, with thieves knowing exactly what to steal and where to find it.

Cultural historians fear that some of the goods may already have been absorbed into the trafficking rings that transport antiquities through middlemen to collectors in Europe, the US and Japan.

Three of the most important antiquities from the National Museum of Baghdad, including a 5,000-year-old vase, appeared to have been specifically targeted. "It looks as if part of the looting was a deliberate, planned action," said McGuire Gibson, a University of Chicago professor and president of the American Association for Research in Baghdad.

"They were able to take keys for vaults and take out important Mesopotamian materials put in safes. I have a suspicion it was organised outside the country, in fact I'm pretty sure it was," he said.

A huge amount of valuable property was stolen after the 1991 Gulf War and only a tiny amount later recovered. A handful of items were spotted on the art markets in the UK, and the British Museum arranged for them to be returned to Iraq.

The extent to which some gangs were prepared to go to secure plunder was highlighted by an attack on one Iraqi expert, who was beaten over the head outside his home. He had been put in charge of a newly uncovered site prior to the US-led invasion, according to Dr Dominique Collon, an expert on antiquities from the region at the British Museum.

The Unesco experts have already started putting together a plan to try to recover some of the missing antiquities.

The Art Newspaper, an international publication, has posted pictures from a 1975 catalogue of 300 of the most prominent exhibits from the museum in Baghdad on its website. The newspaper said it was not yet clear which of the items had been stolen, broken or escaped the looting.

Officials at the Unesco meeting confirmed that information was sketchy. The experts, who included Iraqi art officials, said some of the most valuable pieces had been placed in the vault of the national bank after the 1991 Gulf War, but they did not know if they were still there.

Koichiro Matsuura, the director-general of Unesco, called for a UN resolution to impose a temporary ban on trade in Iraqi antiquities. Such a resolution would also call for the return of such items to Iraq, he said.

The London art trade said some of the artefacts were likely to be smuggled overland into Syria and Jordan before being more widely distributed.

James Ede, the chairman of the Antiquities Dealers' Association, said controls were more stringent than before the last war. "Before, they ended up in America, Switzerland and France, but Europe as a whole has tightened up. It's quite difficult to sell anything that doesn't have reasonable provenance."

US forces have been criticised for failing to protect some of the most precious sites despite repeated warnings from academics before the invasion of the dangers of looting.

At a briefing in Kuwait on 5 April, Major Christopher Varhola, a cultural anthropologist attached to the US Army, said that US military planning had included protecting cultural sites.

"All around Iraq, there are a number of museums, in particular the National Museum of Baghdad, that hold priceless materials," he said. "The US military is eager to co-ordinate with any organisation dedicated to the task of preservation which transcends military and operational necessity."

Unesco called for the formation of a national heritage police to watch over cultural sites in Iraq, although the scale of the task was highlighted by a British Museum expert, who described the whole country as a "vast archaeological site".