Egypt has put out an international alert after a priceless statuette of King Tut's sister was stolen from the Mallawi City Museum in central Egypt.
Hundreds of other articles were stolen during August clashes between Islamist supporters and police.
"A Daughter of the Pharaoh Akhenaten" dates back to the 14th century BC. It is feared that the precious carving will be sold on the international black market, along with the other missing items.
During the fighting, which killed hundreds of people, looters made away with numerous items and smashed and burned what they could not carry.
Angry crowds had apparently surrounded the museum and law enforcement was helpless to stop them from entering.
About 1,000 items were stolen during the ransacking, with 600 having been returned so far.
The UN agency responsible for world heritage and culture said the loss was tragic and that it was aiding in efforts to recover the items.
“This constitutes irreversible damage to the history and identity of the Egyptian people,” said Irina Bokova, UNESCO director general.
“UNESCO is continuing to work closely with the Egyptian authorities as well as with its partners … to fight, by all possible means, the illicit trafficking of these stolen cultural objects,” the organization said in a statement.
It is believed that the looting could have been coordinated during the chaos.
A collection of Greek gold coins and statues of ibises were stolen and have not been seen again.
Since the 2011 uprising, numerous sites and museums have been pillaged by looters looking to cash in on the lawlessness that has pervaded Egypt after the fall of former autocrat ruler Hosni Mubarak.