The Islamic State (ISIS/ISIL) seized nearly “one-third” of the ancient city Palmyra in Syria after heavy clashes with Bashar al-Assad regime forces and their allies, reported a U.K.-based monitor group.

Palmyra, known as Tadmor in Arabic, is home to a UNESCO World Heritage site, which is at risk of being looted and destroyed by the jihadist group. ISIS has already looted and destroyed ancient sites in Iraq.

Maamoun Abdulkarim, Syria’s antiquities chief, warned that ISIS would destroy the UNESCO historic site if it took control of the ancient ruins.

The Daily Mail reported that ISIS has already “captured large parts of the UNESCO World Heritage site.”

The U.K.-based Syrian Observatory for Human Rights (SOHR), which monitors the Syrian civil war using a network of sources on the ground, reported Wednesday that ISIS had captured “about one-third of the city after the ongoing and violent clashes with the regime forces and allied militiamen.”

Both sides reportedly suffered casualties during the fight, which included mutual bombardments, noted the Observatory.

Rami Abdel Rahman, the Observatory’s chief, said ISIS entered Palmyra through the north “without their vehicles” after heavy fighting on the northern edges of the city, reported Agence France-Presse (AFP).

Quoting Rahman, AFP added, “They seized a state security building and fanned out across northern districts as Assad regime forces fled.”

Mohammad Hassan Homsi, an activist originally from Palmyra, told AFP that the Syrian government troops fled after ISIS seized the state security building and “headed to the military intelligence headquarters near the ruins.”

News reports from two days ago claimed that Syrian government troops pushed ISIS back from the historical ruins and museum facilities in Palmyra.

Predictably, noted The Telegraph, Syrian state television denied reports that ISIS had breached Palmyra’s defenses.

It reported that “armed forces targeted [Islamic State] terrorist groups north of Palmyra, and blocked their infiltration of the northern parts of the city.”

Referring to the ancient monuments and ruins in Palmyra, the Daily Mail noted, “The militants see such sites as targets because of their desire to wipe out all traces of ‘non-Islamic’ history and what they regard as the idolatrous antiquities, icons and carvings they have.”

“The situation is very bad. If only five members of ISIS go into the ancient buildings, they’ll destroy everything,” Syria’s antiquities chief told AFP, calling for international action to save Palmyra.

Palmyra’s UNESCO World Heritage site includes 2,000-year-old temples and colonnaded streets, according to AFP. It is located in the city’s south-west.

Syria’s antiquities chief said, “Hundreds of statues and ancient artifacts from its museum have already been transferred out of the city,” adding that “many others–including massive tomb–could not be moved,” reported AFP.