Islamic State group jihadists have blown up the Arch of Triumph in Syria's Palmyra, the country's antiquities chief said on Monday, stepping up their campaign of destruction at the world heritage site.

The move was seen as the latest salvo in the propaganda war being waged by the extremist group which has seized large parts of Syria and neighboring Iraq, committing atrocities including beheadings, rape and mass killings, and destroying archaeological and cultural heritage.

Syrian Antiquities Director Maamun Abdulkarim warned on Monday of a catastrophe unfolding in Palmyra, which the jihadists have been dismantling since capturing it in May.