French President Francois Hollande
French President Francois HollandeFlash 90

French President Francois Hollande on Thursday said the world must respond to Islamic State (ISIS) jihadists' seizure of Palmyra amid fears they could destroy the Syrian city's world renowned ancient monuments.

"We have to act because there is a threat against these monuments which are part of humankind's inheritance and at the same time we must act against Daesh," Hollande said, according to AFP, referring to the Islamic State group by its Arabic name.

"It is really upsetting when a site of such riches which belongs to all of humanity falls into the hands of a terrorist group," he added, as he arrived at an EU-Eastern Partnership summit in the Latvian capital Riga.

The comments came hours after UNESCO warned that the destruction of the ancient city would be "an enormous loss to humanity".

The capture of Palmyra, a 2,000-year-old metropolis from the Roman era, reportedly means ISIS now controls more than half of Syria at a time when it is also expanding its territory in neighboring Iraq.

In Iraq, ISIS has already sparked international outrage when it blew up the ancient Assyrian city of Nimrud and smashed artifacts in the Mosul museum.

The fear now is the extremists will do the same in Palmyra.

AFP contributed to this report.