Islamic State (IS) militants in Syria have entered the Unesco World Heritage site of Palmyra after seizing the town next to the ancientruins, reports say.
Unesco says its destruction would be "an enormous loss to humanity", but no damage has been reported there yet.
IS now control the nearby airport, prison and intelligence HQ, the Syrian Observatory for Human Rights says.
The militants have previously demolished ancient sites in Iraq that pre-date Islam.
The BBC's Jim Muir in Beirut says acute international concern over Palmyra might actually spur the jihadists on to make destroying it a priority, since they delight in challenging and horrifying world opinion.
The ancient ruins are situated in a strategically important area on the road between the capital, Damascus, and the contested eastern city of Deir al-Zour.
Palmyra is also close to oil and gas fields which the Syrian government uses to generate electricity for areas under its control in the west of the country.
Rising out of the desert, the site contains the monumental ruins of a great city, which Unesco and others consider one of the most important cultural centres of the ancient world.
Dating back to the 1st and 2nd Century, when the region was under Roman rule, Palmyra is dominated by a grand, colonnaded street.