The Islamic State extremist group has demolished parts of the temple of Bel, the largest monumental complex in the ancient city of Palmyra, the Syrian Observatory for Human Rights says.
A local activist group says the extremists had blown up the richly-carved main temple building at the centre of the sanctuary.
The force of the blast also caused major damage to the sanctuary's colonnades and fortified outer wall, the Palmyra Coordination activist group told DPA on Sunday.
The temple of Bel stands at the end of the ancient city's column-lined principle street.
Its central shrine area, or cella, was extremely well preserved.
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According to UN cultural agency UNESCO, the temple is "one of the most important religious buildings of the 1st century AD in the East and of unique design".
Islamic State captured Palmyra, an oasis town and former trading centre in Syria's central desert, from government forces in May.
Last week, the extremists demolished the city's temple of Baal Shamin, which was small but also in excellent condition.
Photographs published by the group showed a pile of rubble on the site of that temple.
Its destruction was condemned by UNESCO, which lists Palmyra as a World Heritage Site, as a war crime.
The head of the UN cultural agency, Irina Bokova said Palmyra's art and architecture was "a symbol of the complexity and wealth of the Syrian identity and history".
Two weeks ago, Islamic State beheaded Khaled al-Asaad, who had served as the city's chief archaeologist for some 40 years, and hung his body from a lamp post.
The group, which controls most of eastern Syria and the central desert, has previously destroyed ancient monuments as well as Christian and Muslim holy sites in neighbouring Iraq.
