WASHINGTON (Reuters) - The U.S. government is suing to recover four ancient Syrian artifacts it believes were trafficked by Islamic State, U.S. officials said on Thursday, a tiny fraction of the plundered antiquities likely to have passed through the jihadist group’s hands.

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Islamic State used the mayhem of war to establish a lucrative trade in stolen relics dug up from the territory it controlled in Syria and Iraq, which includes remnants of some of the world’s oldest and most culturally rich civilizations, according to archaeological experts.

U.S. Department of Justice officials filed a complaint in U.S. District Court for the District of Columbia on Thursday seeking the forfeiture of the antiquities, including a gold ring with a carved gemstone, two gold coins, and a neo-Assyrian stone stela, together worth hundreds of thousands of dollars.

The suit, believed to be the first of its kind, is meant to signal to art collectors, dealers, and auction houses around the world “that they’re on notice, that they need to be vigilant, and that they need to take steps to ensure that their purchases aren’t either knowingly or unknowingly supporting ISIL’s terrorist activities,” said Andrew Keller, a counter threat finance official at the State Department, using an acronym for Islamic State.

The FBI warned last year there was evidence that collectors had been offered artifacts plundered by Islamic State.

Unscrupulous dealers can dupe unwitting collectors who often are not antiquities experts, said Robert Wittman, who founded the Federal Bureau of Investigation’s art crime team and now works as a private consultant on art security.

“It’s not so much the collector who’s really creating a problem, it’s the unscrupulous dealer who has an artifact that doesn’t have proper provenance,” Wittman said. “And if there’s a provenance that’s created for it, how is a customs official going to know?”

U.S. officials learned of the items detailed in Thursday’s lawsuit through documents seized in a May 2015 U.S. raid in Syria targeting Abu Sayyaf, a top Islamic State finance official and Tunisian militant whose real name was Fathi ben Awn ben Jildi Murad al-Tunisi.

“During the raid, there was a lot of electronic media that was recovered, and these are not the only four items that were on that media that we are going after,” said Assistant U.S. Attorney Arvind Lal.

U.S. officials on Thursday also released documents seized from the raid that depict a sophisticated system used to process and manage the trade in antiquities. They show that Abu Sayyaf, who led Islamic State’s antiquities division, levied a 20 percent tariff on local merchants who brought the items to Islamic State, and the payments were documented.

Officials said it was difficult to know how many ancient relics have passed through Islamic State control, making it hard to assess the total cultural loss to Syria and Iraq. Syria’s border with Turkey, believed to be a major stop on the smuggling route, is porous, though Turkish officials have seized thousands of antiquities over the last several years, U.S. officials said.

At the height of its territorial reach, U.S. officials estimate, Islamic State controlled about 5,000 archaeological sites, though it has lost significant territory in the last year.

In addition to ransacking sites for loot, Islamic State also destroyed some sites in northern Iraq and Syria, posting photos and videos of fighters destroying pre-Islamic monuments and temples they consider idolatrous.

Islamic State recaptured the ancient Syrian city of Palmyra, a 2000-year-old UNESCO World Heritage Site, this month after losing it in March. The last time it had control over the city, Islamic State blew up two ancient shrines.