The militants have carried out a sustained campaign of destruction against heritage sites in areas under their control in Syria and Iraq, and in mid-August beheaded Palmyra's 82-year-old former antiquities chief.
Syrian antiquities director Maamun Abdulkarim warned of impending catastrophe in the UNESCO-listed world heritage site, which the jihadists have been dismantling since capturing the ruins in May.
"This is a systematic destruction of the city. They want to raze it completely," Abdulkarim told AFP.
Known as the "Pearl of the Desert", the ancient oasis town of Palmyra situated about 210 kilometres northeast of Damascus became famous as a stopping point for caravans travelling on the Silk Road.
Both the citadel and the ruins are on the UNESCO World Heritage list, and before the war around 150,000 tourists a year visited Palmyra.
ISIS has already destroyed the shrine of Baal Shamin and the 2,000-year-old Temple of Bel, regarded as Palmyra's masterpiece, as part of a campaign to destroy pre-Islamic monuments, tombs and statues it considers idolatrous.
"We have received news from the site that the Arch of Triumph was destroyed yesterday (Sunday). ISIS booby-trapped it several weeks ago," he told AFP.
"We are living through a catastrophe. Since the capture of the city (by ISIS), it has been one shock after another."
The Syrian Observatory for Human Rights monitoring group and local activist Mohammad Hassan al-Homsi backed up the account, saying the arch had been ruined. "The Arch of Triumph was pulverised. ISIS has destroyed it," said Homsi.
