BEIRUT, Lebanon — Another landmark structure in Syria’s ancient city of Palmyra has been deliberately destroyed by Islamic State militants, according to local antigovernment activists and Syrian officials. The building involved this time was a set of arches, erected in the second century.

Since seizing Palmyra from government forces in May, Islamic State fighters have destroyed some of the most beautiful and historically significant monuments in the sprawling oasis city in Syria’s central desert, one of the world’s most renowned archaeological sites.

The latest to fall was the decorative triple arch built by the Romans, which bore ancient inscriptions and stood at the entrance to a grand colonnade.