The self-declared Islamic State has taken control of Palmyra, an ancient city that's on UNESCO's list of World Heritage sites.
Palmyra and Tadmur, the modern town that adjoins it, have been the scene of recent fighting between Syrian government troops and fighters from the Islamic State. Multiple news reports say government troops left the city ahead of an advance by the rebels.
Last week, UNESCO appealed to Syria's warring factions to "make every effort to prevent" Palmyra's destruction. On Wednesday, the organization's head called for a cessation of hostilities.
"I am deeply concerned by the situation at the site of Palmyra," Director-General Irina Bokova said. "The fighting is putting at risk one of the most significant sites in the Middle East, and its civilian population."
Here's how UNESCO describes Palmyra:
"An oasis in the Syrian desert, north-east of Damascus, Palmyra contains the monumental ruins of a great city that was one of the most important cultural centers of the ancient world. From the 1st to the 2nd century, the art and architecture of Palmyra, standing at the crossroads of several civilizations, married Graeco-Roman techniques with local traditions and Persian influences."
The Islamic State has destroyed other pre-Islamic treasures in areas it controls, and it is feared it might do the same in Palmyra.
Control of Palmyra is more than just symbolic. The New York Times notes:
"But for the fighters on the ground, the city of 50,000 people is significant because it sits among gas fields and astride a network of roads across the country's central desert. Palmyra's vast unexcavated antiquities could also provide significant revenue through illegal trafficking.
"Control of Palmyra gives the Islamic State command of roads leading from its strongholds in eastern Syria to Damascus and the other major cities of the populated west, as well as new links to western Iraq, the other half of its self-declared caliphate."