BRUSSELS (AFP) - EU foreign policy chief Federica Mogherini on Thursday warned of potential war crimes in the Unesco World Heritage site of Palmyra after Islamic State in Iraq and Syria (ISIS) fighters seized the ancient Syrian city.

"With the reported occupation of the ancient city of Palmyra by Da'esh, yet again hundreds of people have been killed and thousands more risk to be exposed to arbitrary violent actions and more destructions of cultural sites might be perpetrated," Mogherini said in a statement, using an alternative name for ISIS fighters.

ISIS "mass killings and deliberate destruction of archaeological and cultural heritage in Syria and Iraq amount to a war crime according to the Rome Statute of the International Criminal Court," she said.

ISIS fighters seized full control of Palmyra on Thursday, putting the world heritage site and its priceless artefacts at risk of destruction.

"Palmyra is a symbol of Syria's rich cultural heritage and has been a crossroad of civilisation throughout history," Mogherini said.

The fall of the ancient city comes during an ISIS campaign of so-called "cultural cleansing", with Islamist militants destroying relics all across ancient Mesopotamia.

Mogherini, currently on tour in the Middle East, warned that the destruction of cultural sites in the region could amount to a war crime.

In addition, the EU "has taken all the appropriate steps ... to prevent the illegal trade of cultural property," Mogherini said, adding that such illicit traffic of cultural artefacts, directly contributes to the financing of ISIS.