Story highlights UNESCO says the destruction of the temple is "a new war crime"

The Temple of Baalshamin is reported to have been rigged with explosives

ISIS seized the Palmyra ruins in Syria in May, prompting fears for the ancient site

(CNN) The burgeoning list of appalling acts by ISIS has grown even longer: The Islamic extremist group has blown up a nearly 2,000-year-old temple in the historic ruins of Palmyra, Syria.

UNESCO, the United Nations cultural organization, called the destruction of the Temple of Baalshamin a "war crime."

Maamoun Abdulkarim, Syria's director-general of antiquities and museums, said Sunday that sources in Palmyra informed him that ISIS members rigged the temple with large quantities of explosives and detonated them.

There was uncertainty -- not unusual amid the chaos of the Syrian conflict -- over when exactly the damage was done to the temple, which dates from the first century.

Photos: How ISIS ravaged Palmyra's world treasures Photos: How ISIS ravaged Palmyra's world treasures Before: The ruins of the 2,000-year-old Temple of Baalshamin are seen in Palmyra, Syria, in 2007. The ISIS militant group took over the ancient city last year and razed parts of its The ruins of the 2,000-year-old Temple of Baalshamin are seen in Palmyra, Syria, in 2007. The ISIS militant group took over the ancient city last year and razed parts of its World Heritage Site. Syrian government forces recaptured the ancient city from the terror group in March 2016. Click through to see the landmarks before and after ISIS' occupation. Hide Caption 1 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: A Syrian government soldier walks near what's left of the Temple of Baalshamin on Sunday, March 27. Syrian forces retook the city days before, but damage had already been done by ISIS. UNESCO says it plans to evaluate the extent of A Syrian government soldier walks near what's left of the Temple of Baalshamin on Sunday, March 27. Syrian forces retook the city days before, but damage had already been done by ISIS. UNESCO says it plans to evaluate the extent of Palmyra's damage soon. Many of the structures -- which date from the first and second centuries and marry Greco-Roman techniques with local traditions and Persian influences -- remain in place, bolstering hopes that ISIS didn't completely raze the world heritage site. Hide Caption 2 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before: The Arch of Triumph in 2006. Hide Caption 3 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: The Arch of Triumph on March 27. Hide Caption 4 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before: The Temple of Bel in 2008. Hide Caption 5 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: The Temple of Bel on March 27. Hide Caption 6 of 15 Photos: How ISIS ravaged Palmyra's world treasures Destroyed statues are seen inside the damaged Palmyra Museum on March 27. Hide Caption 7 of 15 Photos: How ISIS ravaged Palmyra's world treasures Many statues in the museum had their faces chipped off -- in keeping with strict Sharia interpretations of the depiction of human forms. Hide Caption 8 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before ISIS invaded, authorities took what they could from the museum. But larger items and those fixed to walls had to be left behind. Hide Caption 9 of 15 Photos: How ISIS ravaged Palmyra's world treasures Destroyed statues inside the Palmyra Museum. Hide Caption 10 of 15 Photos: How ISIS ravaged Palmyra's world treasures The Syrian directorate-general of antiquities and monuments was positive that the condition of artifacts meant that they could be restored and their "historic value" returned, according to a translation of an article on the department's website. Hide Caption 11 of 15 Photos: How ISIS ravaged Palmyra's world treasures ISIS took over Palmyra in May. Hide Caption 12 of 15 Photos: How ISIS ravaged Palmyra's world treasures By June, ISIS began destroying historical sites. Hide Caption 13 of 15 Photos: How ISIS ravaged Palmyra's world treasures Damage inside the Palmyra Museum. Hide Caption 14 of 15 Photos: How ISIS ravaged Palmyra's world treasures Palmyra was a caravan oasis when Romans overtook it in the mid-first century. In the centuries that followed, the area "stood at the crossroads of several civilizations" with its art and architecture mixing Greek, Roman and Persian influences, according to UNESCO. Hide Caption 15 of 15

Abdulkarim said it was blown up Sunday. But the UK-based Syrian Observatory for Human Rights, which monitors the conflict, reported Sunday that its sources said ISIS blew up the temple about a month ago.