Particularly vulnerable to the fighting have been citadels and castles, which were often built on high points so that soldiers in ancient times could spot the approach of their enemy. The same holds true today and rebels periodically claim sites, such as the famous crusaders’ castle, the Krak des Chevaliers. Then the Syrian Army fights to get it back, almost inevitably damaging the ancient walls, roofs and carvings. Sometimes sites change hands two or three times, each time suffering more damage from both sides.

The looting and pillaging has occurred largely in rebel-held areas, but also in contested places. When the fighting began and the foreign archaeologists left, the local guards, who often were no longer being paid, left their posts. Local residents, who were jobless, then often dismantled the structures where archaeologists had stored as-yet unlabeled finds, such as pottery shards and small artifacts; they broke into on-site museums and stole the windows and doors, the wood used in the buildings’ construction, the electrical wire and even pipes, according to the archaeologists.

The archaeologists said they did not blame the residents. “These are poor people in a crisis; one is worried for them,” said Agnès Vokaer, the field director of the Belgian archaeological team at Apamea, one of the largest Roman and early Christian sites in Syria. “There are no telephones, no electricity, there is no fuel for running agricultural machinery, there is no more food.”

The archaeologists are far more disturbed about what happened next. Foreign fighters soon arrived, and with them criminals who took a more ruthless approach. By late 2011 or early 2012, depending on the site, they were working with mechanized digging equipment and jackhammers and had a seemingly clear idea of what they wanted, according to residents. They set up armed guards as lookouts while the illegal excavators went to work.

“We have approximately 1,000 people working every day to find coins, objects, to find something to sell,” said Mr. Leriche of his site at Douros Europos, adding that the thieves worked with metal detectors burrowing into the ground whenever there were signs of metal.

They are digging as well for mosaics, said Ms. Hassan of Unesco.

Lebanon has intercepted 86 mosaics that were looted from Syria and returned them, but that is a tiny percentage of what archaeologists believe has been taken, she said.