The 2,000-year-old temple was part of the remains of the ancient caravan city of Palmyra in central Syria, seized by IS in May.
The news of the latest destruction at Palmyra came just days after IS released propaganda images purportedly showing militants blowing up another Palmyra temple, the 2,000-year-old Baalshamin dedicated to the Phoenician god of storms and fertilizing rains.
The UN cultural agency Unesco, which has designated Palmyra as a world heritage site, called the destruction of the Baalshamin temple a war crime.
Earlier this month, relatives and witnesses said that IS militants had beheaded Khaled al-Asaad, an 81-year-old antiquities scholar who devoted his life to understanding Palmyra.
The Islamic State group, which has imposed a violent interpretation of Islamic law across its self-declared “caliphate” straddling Syria and Iraq, says such ancient relics promote idolatry.
It already has blown up several sites in neighbouring Iraq, and it is also believed to be selling looted antiquities.
A Palmyra resident, who goes by the name of Nasser al-Thaer, said IS militants set off a huge blast at 1:45 pm on Sunday.
“It is total destruction,” he said of the scene of the explosion. “The bricks and columns are on the ground.”
“It was an explosion the deaf would hear,” he added. The resident said only the outer wall surrounding the temple remains.
The Britain-based Syrian Observatory for Human Rights, which has a network of activists in Syria, said the temple was damaged. It did not provide details.
The temple, consecrated to the Semitic god Bel, had been well-preserved and was a source of much pride for Syrians. It was consecrated in 32AD.
It stood out among the ruins not far from the colonnades of Palmyra, which is affectionately known by Syrians as the “Bride of the Desert”.
Earlier on Sunday, IS fighters pushed into a large district in southern Damascus, clashing with rival militants just a few miles from the centre of the Syrian capital, the extremist group and Syrian activists said.
More than two dozen militants were killed in the clashes on the edges of the Qadam neighbourhood, said the Observatory.
The pro-IS Aamaq News Agency reported that IS fighters seized half of Qadam.
The Syrian Observatory’s Rami Abdurrahman said IS fighters were holding two streets and that fighting was continuing.
IS supporters posted propaganda pictures claiming to show IS fighters advancing in the narrow streets of Qadam. The authenticity of the images could not be confirmed independently.