Professional thieves appear to have slipped in among the bands of looters in Iraqi museums, curators said Tuesday as they urged U.S. authorities to tighten border security and stop the flow of stolen treasures.

Gen. Tommy Franks, commander of coalition forces in Iraq, said Monday that the looting in Baghdad did not appear to be the work of organized thieves.

But Donny George of the Iraqi National Museum said Tuesday that glass cutters and bunches of keys found amid the museum's debris suggested at least some of the items had been snatched by robbers with expert eyes.

He said some of the looters "had come into the museum with some kind of preparation to take objects they had in mind.

"They knew what those pieces were. ... In one of the corridors we had a fake statue. They never touched it."

At a meeting in London, the world's top curators said antiquities are still being smuggled out of the country, three weeks later, and passed a resolution backing George's call for the United States to secure Iraq's borders.

"American control at the border is almost zero," said George, research director at the museum, whose collection of 170,000 pieces spanning half a million years was pillaged after the collapse of Saddam Hussein's regime this month. "Anyone can take anything and go out ... the bleeding of antiquities is still going on."

Iraq's museums held priceless, millennia-old collections from the Assyrian, Sumerian and Babylonian cultures. Ancient Mesopotamia -- modern-day Iraq -- was the cradle of urban civilization.

After the fall of Saddam's government, looters stole and smashed priceless archaeological treasures from the National Museum in Baghdad and other museums and libraries.

Many Iraqis criticized U.S. troops for doing little to stop the theft. George said a staff member at the museum had begged U.S. troops to park their tanks nearby to discourage looting. But nothing was done for several days, he said.

Neil MacGregor, director of the British Museum, also said it was "extraordinary" that American troops did not prevent the theft.

"This is without question the greatest disaster to a national collection since the Second World War, without question," MacGregor told British Broadcasting Corp. radio.

MacGregor said after Tuesday's meeting -- attended by delegates from the Louvre in Paris, New York's Metropolitan Museum of Art, Russia's Hermitage and the Berlin Museums -- that the British Museum would coordinate offers of help to Iraq and oversee the training of experts to be sent to the country.

At the meeting, the representatives and the U.N. cultural organization, UNESCO, also urged the U.N. Security Council to ban the trade in Iraqi artifacts.

UNESCO said its director-general, Koichiro Matsuura, would meet Wednesday with U.N. Secretary-General Kofi Annan to press the case for a ban.

UNESCO said it would soon send a panel of experts to Iraq to create a database of the country's missing artifacts. The list would be shared with police around the world through Interpol, said the organization's assistant director-general, Mounir Bouchenaki.

George said American officials had arrived to help the museum take stock of its missing objects, and that many small items had been brought back -- some in pieces -- after appeals on radio and from religious leaders.

But he said he feared many of the museum's masterpieces would never be recovered. They include the sacred Sumerian "Warka vase" from 3100 B.C., and an inscribed copper statue base from 2250 B.C.

Other experts shared that concern.

"They are too well known to appear in museums," said Dominique Collon of the British Museum. "I fear they are in private collections and we'll never see them again."