BEIRUT: The hardline Islamic State group has destroyed part of an ancient temple in Syria's Palmyra city, a group monitoring the conflict said on Sunday.

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 Islamic State has targeted in Palmyra this month. The group detonated explosives in the ancient Baal Shamin temple on Aug. 25, an act that 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.

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

The group, which has proclaimed a caliphate in territory it holds across Syria and Iraq, has regularly demolished monuments it considers sacrilegious as well as carrying out mass killings.

This month the group beheaded the 82-year-old Syrian archaeologist who had looked after Palmyra's ruins for four decades, and hung his body in public, according to Syria's antiquities chief.