ISIS has destroyed part of Palmyra’s Temple of Bel, which is considered the most important temple at the ancient Syrian site, activists and witnesses say.
The extent of the damage to the 2,000-year-old Temple of Bel is not clear but local residents have described being shaken by a large explosion.
The reports come a week after ISIS blew up another Palmyra temple.
ISIS seized control of Palmyra in May, sparking fears for the site.
The world-famous Greco-Roman ruins are in the desert north-east of the Syrian capital, Damascus.
“It is total destruction,” one Palmyra resident told the Associated Press news agency.
“The bricks and columns are on the ground.”
“It was an explosion the deaf would hear,” he went on, adding that only the wall of the temple remains.
The temple was dedicated to the Palmyrene gods and was one of the best preserved parts of the site.
It was several days after the initial reports of the destruction of another part of the site, the Temple of Baalshamin that ISIS itself put out pictures showing its militants blowing up the temple.
Satellite images have confirmed the destruction.
For the extremists, any representation implying the existence of a god other than theirs is sacrilege and idolatry.
Earlier this month ISIS murdered 81-year-old Khaled al-Asaad, the archaeologist who had looked after the Palmyra ruins for 40 years.
Khaled al-Asaad’s family told Syria’s director of antiquities that he had been beheaded.
UNESCO Director General Irina Bokova praised Khaled al-Asaad, saying ISIS “murdered a great man, but they will never silence history”.
The ancient city of Palmyra is a UNESCO World Heritage site and was a major tourist attraction before Syria descended into civil war.
UNESCO has condemned the deliberate destruction of Syria’s cultural heritage as a war crime.
The modern city of Palmyra – known locally as Tadmur – is situated in a strategically important area on the road between the Syrian capital, Damascus, and the eastern city of Deir al-Zour.
ISIS has used Palmyra’s theatre to stage the public execution by children of more than 20 captured Syrian army soldiers.
The militant group has ransacked and demolished several similar sites in the parts of neighboring Iraq which they overran last year, destroying priceless ancient artifacts.
The UN estimates that over 250,000 people have been killed in Syria since the war began in 2011.
Over 4 million people have fled Syria and 7.6 million are displaced inside the country.