The minaret of Aleppo's ancient Umayyad mosque has been destroyed, Syrian state media and activists say, with the regime and the opposition blaming each other.
The mosque, in Aleppo's UNESCO-listed Old City, has been the centre of fighting for months and had already suffered extensive damage.
On Wednesday, as reports broke of the minaret's destruction, activists uploaded video shot at the scene, but there was no video immediately available showing the moment of the blast that caused the collapse.
State media said rebel fighters from the group Jabhat al-Nusra blew up the minaret, and accused the group classed by the US as a "terrorist" organisation of seeking to blame loyalist forces.
But rebels, the opposition and activists all said the army was responsible.
"Tanks began firing in the direction of the minaret until it was destroyed," one rebel said in a video posted on YouTube, insisting rebel snipers were not stationed inside the minaret.
"We were afraid that it would be targeted," he said.
The ancient mosque has fallen in and out of rebel hands several times.
It was originally built in the 8th century but was apparently destroyed and then rebuilt in the 13th century.
It has recently fallen back into rebel hands, but has been left pockmarked by bullets and stained with soot.
Antique furnishings and intricately sculpted colonnades have been charred, valuable Islamic relics ransacked and ancient artefacts - including a box purported to contain a strand of the Prophet Muhammad's hair - looted.
The UN's cultural agency, UNESCO, has warned that the civil war poses a grave threat to the country's rich cultural heritage, and appealed to all parties involved in the conflict to protect important sites.
Both rebels and regime forces have turned some of Syria's significant historic sites into bases, including citadels and Turkish bath houses, while thieves have stolen artifacts from museums.
Five of Syria's six World Heritage sites have been damaged in the fighting, according to UNESCO.
Looters have broken into one of the world's best-preserved Crusader castles, Crac des Chevaliers, and ruins in the ancient city of Palmyra have been damaged.