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The United States handed back to Iraq on Wednesday antiquities it said it had seized in a raid on ISIS fighters in Syria, saying the haul was proof the militants were funding their war by smuggling ancient treasures.

The Iraqi relics were captured by U.S. special forces in an operation in May against an Islamic State commander known as Abu Sayyaf. They included ancient cylindrical stamps, pottery, metallic bracelets and other jewelry, and glass shards from what appeared to be a colored vase.

The haul also included early Islamic coins. ISIS, a hardline Sunni Islamist group, has ransacked some of the greatest archaeological sites in northern Iraq, posting video footage of fighters destroying pre-Islamic monuments they consider idolatrous.

Iraqi officials have been unable to verify the full extent of damage at the sites under ISIS control, but they have said that footage of the destruction was published in part to distract attention from the fact ISIS is smuggling antiquities to raise cash.

"This is the first tangible evidence that Daesh are selling artifacts to fund their activities," U.S. ambassador Stuart Jones, said, referring to Islamic State by its Arabic acronym.

Read More: ISIS Looting Syrian Sites 'Into Oblivion,' as Fears Mount for Palmyra

"Their goal is to sell these antiquities on the global black market," he told reporters at Baghdad's national museum where the items were handed over.