BEIRUT (AP) — Islamic State militants destroyed parts of the second-century Roman theater in Syria’s historic town of Palmyra and the site’s landmark ancient Tetrapylon, the government and opposition activists said Friday.

It was the extremist group’s latest attack on world heritage, an act that the U.N. cultural agency called a “war crime.” A Syrian government official said he feared for the remaining antiquities in Palmyra, which IS recaptured last month.

Palmyra, a UNESCO world heritage site that once linked Persia, India, China with the Roman empire and the Mediterranean area, has already seen destruction at the hands of the Islamic State group.  The ancient town first fell to IS militants in May 2015, when they held it for 10 months. During that time, IS damaged a number of its relics and eventually emptied it of most of its residents, causing an international outcry.

Palmyra fell again to the group last month, only nine months after a Russian-backed Syrian government offensive was hailed as a significant victory for Damascus.