The U.N. cultural agency says six historical sites in Syria are in imminent danger because of the country's civil war and is urging international support to protect them.

UNESCO said in a statement Thursday that its World Heritage Committee says the sites are "of outstanding universal value for humanity as a whole."

Some significant mosques, citadels and souks have gotten caught in the crossfire between regime forces and rebels, and illicit trade in Syria's rich archaeological heritage has flourished during the two-year war.

The sites on the UNESCO committee's danger list include the historical centers of Damascus, Bosra and Aleppo, as well as the desert oasis of Palmyra, a cultural center of the ancient world.