The Islamic State released photos showing the destruction of six priceless artifacts from the ancient city of Palmyra. The photos show jihadis taking a sledgehammer and smashing the historic treasures, including one dating from the second century.

Jihadis took sledgehammers to the relics, smashed them to pieces and then lashed the man who allegedly smuggled the artifacts in a public square full of onlookers, they announced on social media Thursday.

One-fifth of Iraq's approximately 10,000 world-renowned cultural heritage sites are under the Islamic State's control and most have been heavily looted, Irina Bokova, the head of the U.N. cultural agency UNESCO, warned experts in London Thursday. Some Syrian sites have been so badly ransacked that experts say they no longer have historical or archaeological value.

Related: Razed, bulldozed and destroyed: A list of ISIS' obliteration of history

"Violent extremists don't destroy [heritage] as a collateral damage, they target systematically monuments and sites to strike societies at their core," Bokova said Wednesday.

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