But the head of President George W. Bush's cultural advisory committee, stepped down in protest over the US failure to stop the pillage, adding to international calls for action to protect Iraq's heritage.

Federal Bureau of Investigation director Robert Mueller said agents had been sent to Iraq to investigate the looting and to give general help to efforts to improve security in Iraqi cities where there have been widespread troubles since the fall of the Saddam Hussein regime.

"We are firmly committed to doing whatever we can to secure these treasures for the people of Iraq," Mueller told a news conference, without saying how many agents were involved.

But the chairman of the President's Advisory Committee on Cultural Property quit after eight years in the post, saying the devastation of a Baghdad museum was a "tragedy" and a result of US negligence.

"The reports in recent days about the looting of Iraq's National Museum of Antiquities and the destruction of countless artifacts that document the cradle of Western civilization have troubled me deeply, a feeling that is shared by many other Americans," said Martin Sullivan in a letter.

A source close to the committee told AFP that another committee member, Gary Vikan, was also stepping down.

Baghdad's museum, which housed a major collection of artefacts from early Mesopotamian civilizations, was ransacked on Friday in the upheaval following US troops' entry into the city.

Critics have faulted US forces for failing to halt the pillaging of the capital and other Iraqi cities after Saddam Hussein's regime collapsed.

The US government has offered rewards for the return of the items or assistance in their recovery, but US Defense Secretary Donald Rumsfeld said Tuesday: "To the extent it happens in a war zone, it's difficult to stop."

The White House said Thursday it hoped the rewards would bring the return of the stolen cultural treasures.

"It is unfortunate that there was looting and damage done to the museum, and we have offered rewards ... for individuals who may have taken items from the museum to bring those back, and we are hopeful that will happen," spokeswoman Claire Buchan told reporters.

"The United States, in liberating Iraq, worked very hard to protect the infrastructure of Iraq and to preserve it and the valuable resources of Iraq for the people of Iraq," said Buchan.

Much of the looting was carried out by organised gangs, according to experts at a United Nations conference in Paris.

Among items lost was a collection of around 80,000 cuneiform tablets that contain examples of the some of the world's earliest writing. A 5,000 year-old Sumerian alabaster vase -- known as the Warka vase -- also disappeared.

The meeting of 30 experts at the UN Educational Scientific and Cultural Organisation headquarters was called to take stock of the damage to Iraq's heritage and recommend ways of safeguarding what remains.

"It looks as if at least part of the theft was a very deliberate, planned action," said McGuire Gibson, of Chicago University's Oriental Institute, president of the American Association for Research in Baghdad.

"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," he said.

Students at the University of Chicago's Oriental Institute, a center of research on the region's antiquities, have spent hours this week transferring images of antiquities in old Iraqi museum catalogues to the web, to enable border guards, art dealers and others to more easily identify them.

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