The Islamic State group has blown up a famed temple in Syria's ancient Palmyra, in an act the UN condemned as a war crime and an "immense loss" for humanity. The destruction of the Baal Shamin temple, considered the second-most significant in ancient Palmyra, raised concerns for the rest of the UNESCO World Heritage site. It comes only days after IS beheaded the 82-year-old retired chief archaeologist of Palmyra, sparking widespread revulsion. "This destruction is a new war crime and an immense loss for the Syrian people and for humanity," said Irina Bokova, the head of the UN cultural watchdog UNESCO, calling for the perpetrators to be held accountable. "Daesh (IS) is killing people and destroying sites, but cannot silence history and will ultimately fail to erase this great culture from the memory of the world," Bokova said in a statement. Syria's antiquities chief Maamoun Abdulkarim told AFP the temple was destroyed on Sunday. "Our worst fears are sadly being realised," Abdulkarim said. Famed for its well-preserved Greco-Roman ruins, Palmyra was seized from government forces in May, prompting concerns IS might destroy it as it has other heritage sites in parts of Syria and Iraq under its control. Initially most of Palmyra's best-known sites were left intact, though there were reports IS had mined them and the group reportedly destroyed a well-known statue of a lion outside the city's museum. "Daesh placed a large quantity of explosives in the temple of Baal Shamin today and then blew it up," Abdulkarim said late Sunday. "The cella (inner area of the temple) was destroyed and the columns around collapsed," he said. The Syrian Observatory for Human Rights, a Britain-based group that monitors the country's civil war, confirmed the destruction of the temple. But the Observatory said Baal Shamin had been destroyed a month ago - a discrepancy which could not be immediately explained as information on Syria's civil war is often unclear. IS captured Palmyra on May 21, sparking international concern about the fate of the heritage site described by UNESCO as of "outstanding universal value". Baal Shamin was built in 17 AD and expanded under the reign of Roman emperor Hadrian in 130 AD. Known as the "Pearl of the Desert", Palmyra is an oasis town about 210 kilometres (130 miles) north-east of Damascus and was a stopping point for caravans travelling on the Silk Road and between the Gulf and the Mediterranean. Before the arrival of Christianity in the second century, Palmyra worshipped the Semitic god Bel, whose temple at Palmyra is considered the city's most significant, along with the sun god Yarhibol and lunar god Aglibol. Before the Syrian conflict erupted in March 2011, more than 150,000 tourists visited Palmyra every year.