ISIS has apparently managed to support its war effort by selling stolen antiquities for millions of pounds to western collectors as well as U.K. citizens.

Willy Bruggeman, president of the Belgian federal police council and former deputy director of Europol, said the terrorist group managed to get its hands on stolen stonework and paintings by working with middle men before declaring their Islamic State, according to Breitbart News.

"[ISIS] are now using their own networks to come into contact with the final buyers," the senior police officer said. "When I was working on these cases at Europol, you could see many levels of transactions in this area - now they want to have a one-to-one relationship with the collectors."

Different terrorist organizations have looted more than 1,000 historical sites for financial gain, including Syrian cities like Aleppo, Homs, Apamea and Dura-Europos. Less than 1 percent of stolen artifacts known to have been taken have been recovered.

The report follows claims made earlier this year by Iraqi intelligence that ISIS made 23 million pounds from selling artifacts obtained near al-Nabek, which contains a large amount of early Christian sites, Breitbart News said.

Western authorities have been having trouble estimating how many relics are being smuggled out of the Islamic State since such trades take place a good distance from licensed operations.

Selling stolen artwork is only one of the ways ISIS makes money, with other methods including stealing oil, racketeering and holding hostages for ransom, Breitbart News said. Despite the success of these practices, money is only one motivation for the group.

"You also see a kind of cultural cleansing that undermines the morale of the communities they invade," Bruggeman said.

Authorities have yet to trace artifacts that are being smuggled in Britain, but the U.S. recovered two sets of antiquities believed to have come from sites under ISIS control, and German authorities are currently investigating another case of smuggling.