ISIS militants in Syria and Iraq are engaged in the "most brutal, systematic" destruction of ancient sites since World War II, the head of the U.N. cultural agency said Friday, a stark warning that came hours after militants demolished a monastery with ancient foundations in central Syria. The world's only recourse is to try to prevent the sale of looted artifacts, thus cutting off a lucrative stream of income for the militants, UNESCO chief Irina Bokova told The Associated Press. Recent attacks have stoked fears that ISIS is accelerating its campaign to demolish and loot heritage sites.On Friday, witnesses said the militants bulldozed St. Elian Monastery which houses a 5th century tomb and served as a major pilgrimage site. Days earlier, ISIS 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, ISIS 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. "We haven't seen something similar since the Second World War," Bokova said of the scope of the ISIS campaign against ancient sites. "I think this is the biggest attempt, the most brutal systematic destruction of world heritage." Bokova said recent images of archaeological sites under ISIS control in Iraq and Syria show signs of widespread illegal digging and looting. "If you look at the maps, the photos, the satellite pictures of it, you will not recognize one place," she said. "It is just hundreds of holes all around them." There is very little the world can do to stop the extremists from inflicting more damage, she said, but stopping the trafficking in artifacts must be a priority. Bokova spoke hours after ISIS posted photos on social media showing bulldozers destroying the St. Elian Monastery near the town of Qaryatain in central Syria. The group had captured the town in early August.A Qaryatain resident who recently fled to Damascus said militants leveled the shrine and removed church bells. The man, who spoke on condition of anonymity for fear his relatives still in Qaryatain might be harmed, called on the United Nations to protect Christians and Christian sites. Osama Edward, the director of the Christian Assyrian Human Rights Network, said shelling of the area by Syrian government troops over the past two weeks had already damaged the monastery. "ISIS continued the destruction of the monastery," said Edward. A Catholic priest, the Rev. Jacques Mourad, who had lived at the monastery, was kidnapped in May and remains missing. According to Edward, Mourad sheltered both Muslim and Christian Syrians fleeing the fighting elsewhere in Homs province. Activists said that shortly after capturing Qaryatain, ISIS abducted 230 residents, including dozens of Christians. Activists said some Christians were released, but the fate of the others is still unknown. Bokova said in a statement that "the intentional targeting and systematic destruction of the cultural heritage of Syria is reaching unprecedented levels" and that the militants' campaign "testifies of an ideology of hatred and exclusion."In another attack, ISIS militants beheaded Palmyra scholar Khaled al-Asaad on Tuesday, hanging his bloodied body from a pole in the town's main square. Al-Asaad, a long-time site director, had refused to leave Palmyra after it was overrun by ISIS. Bokova told AP that she believes al-Asaad was "brutally murdered" because he refused to divulge where authorities had hidden treasures secreted out of Palmyra before the ISIS takeover. She would not say whether UNESCO was aware of where the artifacts were taken, saying only "we hope they are in safe places." She recalled her first visit to Palmyra before the outbreak of the conflict, with al-Asaad escorting her. "He introduced me to this beautiful Venice of the desert, as it was called," she said. "We walked through the colonnades, more than a kilometer of beautiful colonnades." Palmyra has remained largely intact, but Bokova said "we know that some of the destruction is starting." "The drama ... and the tragedy, I think is that we don't know what will happen tomorrow," she said. "The fact that Dr. al-Asaad was accused of protecting a place where idolatry is being practiced shows that unfortunately this (destruction) may not stop." Bokova said she is worried about most of the ancient sites in Syria, with some of the damage, including to the Crusader castle of Crac de Chevaliers, caused by fighting. "Nothing is safe," she said, speaking on the sidelines of a youth conference in Madaba, Jordan. Archaeological sites in northern Iraq have also been hard-hit. The destruction there, including smashing ancient statues with a sledgehammer, is a "huge tragedy for all of humanity," Bokova said. She said the international community is increasing efforts to halt the trade in looted antiquities from Syria and Iraq, noting that the U.N. Security Council in February adopted legally binding measures to prevent trafficking. International agencies, including Interpol, are sharing information, and UNESCO has appealed for member states to sharpen legislation. "We have responses from more than 30 countries, and I can tell you that in each of these countries, there is some new measure introduced," she said. She said that the system is starting to work, but that a long road lies ahead.