Amman, Jordan: Islamic State extremists in Syria and Iraq are engaged in the "most brutal, systematic" destruction of ancient sites since World War II, the head of the UN cultural agency said on Friday – a stark warning that came hours after militants demolished a 1500-year-old monastery in central Syria.

The world's only recourse is to try to prevent the sale of looted artefacts, thus cutting off a lucrative stream of income for the militants, UNESCO chief Irina Bokova​ told The Associated Press.

This October 26, 2014, satellite image provided by UNESCO shows levees and roads dug across the Northern Necropolis at Palmyra, Syria. IS fighters overran the historic town in May 2015.

A series of recent attacks has stoked fears that IS is accelerating its campaign to demolish and loot heritage sites. On Friday, witnesses said the militants bulldozed St Elian Monastery in central Syria. Days earlier, IS beheaded an 81-year-old antiquities scholar who had dedicated his life to overseeing the ruins of Palmyra in Syria, one of the Middle East's most spectacular archaeological sites.

Since capturing about a third of Syria and Iraq last year, IS fighters have destroyed mosques, churches and archaeological sites, causing extensive damage to the ancient cities of Nimrud, Hatra​ and Dura-Europos in Iraq. In May, they seized Palmyra, the Roman-era city on the edge of a modern town of the same name.