TURKEY-SYRIA BORDER—In a hotel basement on the Turkish side of this combat-scarred frontier, a group of unlikely warriors is training to fight on a little-known front of Syria’s civil war: the battle for the country’s cultural heritage. The recruits aren’t grizzled fighters but graying academics, more at home on an archaeological dig than a battlefield. For months, they have journeyed across war-torn regions of Syria, braving shelling, smugglers and the jihadists of Islamic State. Their mission: to save ancient artifacts and imperiled archaeological sites from profiteers, desperate civilians and fundamentalists who have plundered Syria’s rich artistic heritage to fund their war effort. Art historians and intelligence officials say that antiquities smuggling by Islamic State has exploded in recent months, aggravating the pillaging by government forces and opposition factions. Looting, often with bulldozers, is now the militant group’s second-largest source of finance after oil, Western intelligence officials say. Dutch archaeologist Rene Teijgeler, left, and Isber Sabrine, a Syrian-born archaeologist based in Barcelona, shown in a market in Gaziantep, Turkey, are helping train the Syrian monuments men how to catalog and preserve sites. Photo: Ayman Oghanna for The Wall Street Journal “What started as opportunistic theft by some has turned into an organized transnational business that is helping fund terror,” said Michael Danti, an archaeologist at Boston University who is advising the U.S. State Department on how to tackle the problem. “It’s the gravest cultural emergency I’ve seen.” In sessions at this secret location, the loose-knit band of academics is being trained how to fight back. They are instructed on how to get to key sites and document both what is there and what is already missing. Another skill: how to hide precious objects that may be at risk of looting and record the GPS locations so they can be retrieved at a later date. The group also uses disguises: posing as antiques dealers to take photographs of looted artifacts. The group is led by a portly, middle-aged archaeologist trained at Damascus University, who with his colleagues operates in secrecy because of the dangerous nature of the work. He likens his group to World War II’s “Monuments Men”: a small group of academics that helped save Europe’s cultural heritage from the Nazis and became the subject of a 2014 Hollywood film starring George Clooney. “It’s dangerous work. We have to get in and out of a site very quickly,” he said, speaking in a dimly lighted basement room used for the training. “The looting has become systematic, and we can’t keep up.” The war in Syria has taken an epic toll, with more than 200,000 people killed since the uprising began in 2011. Alongside the human cost, the cultural damage has mounted. Ancient cities such as Homs and Aleppo have been reduced to rubble. Roman, Greek, Babylonian and Assyrian sites have been destroyed by fighting and looting, and five of the six Unesco World Heritage sites in Syria have been seriously damaged.

Some of the country’s grandest museums have been plundered or are at risk, including the Mosaic Museum in Idlib province, filled with Roman-era works. In the markets in southern Turkish cities like Gaziantep, Roman vases robbed from graves are being sold by the boxload. “We’ve seen a lot of artifacts turning up here…Ottoman-era coffee pots, and older coins and statuettes,” said Harun Unvar, who runs an antiques store in Gaziantep’s old bazaar, as he rejected a Turkish man’s efforts to sell a marble bird’s-head figurine for around $220. “Refugees try to sell small items, but the big stuff is stolen and sold privately for big money.” Market traders say small items such as figurines and carved cylinder seals sell for prices varying from a few dollars to up to several thousand. Buyers range from locals picking up small pieces in Turkish and Lebanese markets to investors and collectors in the West, China and the Persian Gulf, according to antiquities specialists and U.S. officials. In the U.S. alone, government data show the value of declared antiques imported from Syria jumped 134% in 2013 to $11 million. U.S. officials estimate the value of undeclared pieces is many multiples higher. The total volume of illicit trade is impossible to accurately assess but is thought to have mushroomed to more than $100 million a year, according to U.S. officials. A key driver of the dramatic expansion in looting is the rise of Islamic State, also known as ISIS or ISIL. Academics and government officials say the vast majority of the illicit trade is run by the group—whose worldview sanctions destroying artifacts considered idolatrous. In addition to selling oil, the group also makes money from hostage ransoms and racketeering, officials say. In neighboring Iraq, Islamic State is also looting and destroying ancient sites on an alarming scale, according to satellite imagery, archaeologists and government officials. In recent days, local activists reported that the militants destroyed a large portion of the ancient city wall at Nineveh in Iraq, which dates back 2,700 years and was once the capital of the Assyrian Empire. It is unclear whether Iraqi archaeologists are training and deploying into conflict zones to try to limit the damage to their cultural heritage. In Syria, satellite imagery updated in December by the American Association for the Advancement of Science, a Washington-based non-governmental organization, showed how the jihadists are methodically deconstructing and looting historical buildings in their headquarters of Raqqa, a Unesco World Heritage site with ancient shrines that Islamic State regards as sacrilegious. In Islamic State-controlled territory around the Mesopotamian city of Mari, a longtime trade hub founded in 300 B.C., more than 1,300 excavation pits have been dug in the past few months, according to satellite imagery and archaeologists. Researchers from Shawnee State University in Portsmouth, Ohio, say much of the tomb raiding is being done by civilians encouraged by Islamic State leaders, who levy a 20% tax on any sales. Last year, an Iraqi intelligence official claimed Islamic State had made as much as $36 million from looting a single area around al-Nabek, a Syrian city that contains several early Christian sites known for their icons and wall mosaics. Willy Bruggeman, a former deputy director of Europol who is now president of the Belgian federal police council, said Islamic State is using its vast network and social-media savvy to bypass conventional middlemen and reach buyers directly. The looters store the booty in a secret location then circulate the photos directly to buyers in hard copy or via text message or the WhatsApp messaging service, law-enforcement officials say. The Wall Street Journal reviewed cellphone photos of a Bronze Age votive bust, possibly 5,000 years old, looted from Islamic State-controlled territory, being touted for sale to private clients and potentially sold for around $30,000. The limestone statues, depicting in detail the clothing and jewelry of the time, were placed in tombs to accompany the dead to the afterlife. Factions of Islamist fighters immediately take control of trafficking when gaining territory, one smuggler said. “They understand how lucrative this stuff is so they exploit it with sophisticated networks,” said the smuggler from the Turkish border city of Hatay, who identified himself as Ugur. In the city of Manbij, which has become an artifact-trading hub, Islamic State established an office to handle looted antiquities and a market for equipment used in digging, including metal detectors and other remote sensing equipment usually used only by professional archaeologists, said Amr Al Azm, an expert in Syrian antiquities at Shawnee State. Stolen antiquities are usually sold to Islamic State approved dealers, with payments in U.S. dollars. “Once the sales are completed these approved dealers are then given safe passage through ISIS territory,” Mr. Al Azm said.