The hardline Islamic State (IS) group has destroyed part of an ancient temple in Syria's Palmyra city, according to a group monitoring the conflict.

The militants targeted the Temple of Bel, a Roman-era structure in the central desert city, the Syrian Observatory for Human Rights said.

It is the second temple IS has targeted in Palmyra this month. The group detonated explosives in the ancient Baal Shamin temple on August 25, an act cultural agency Unesco has called a war crime aimed at wiping out a symbol of Syria's diverse cultural heritage.

The extent of the damage at the Temple of Bel was not known, the Observatory said, citing its contacts on the ground.

Activists on social media also reported the destruction at the temple, one of Palmyra's most important structures.

IS seized Palmyra in May from government forces in a sudden offensive and is tightly controlling communications in the city, according to activists.