There had been earlier reports of an explosion at the Temple of Bel in Palmyra, which is held by militants from the Islamic State (IS) group.
Syria’s antiquities chief had earlier said the basic structure of the 2,000-year-old site was intact.
But UN satellite analysts Unosat say the imageshows almost nothing remains.
“Unfortunately, the images we acquired do show that the main building of the temple has been destroyed,” Einar Bjorgo, Unosat’s manager, told the BBC early on Tuesday.
He added that a set of columns nearby had also been destroyed.