Syrian authorities say Islamic State militants have advanced to within 2 kilometers of Palmyra, an ancient city with outstanding architectural heritage dating from the first century. The militants have caused significant damage to similar sites they have overrun in Iraq, posting videos of themselves destroying antiquities. UNESCO includes Palmyra on its list of World Heritage sites, describing it as one of the most important cultural centers of the ancient world -- a crossroads for local, Persian, Greek and Roman influences.