DAMASCUS, Syria — Islamic State militants have blown up another major monument in the ancient city of Palmyra, Syria’s antiquities chief said Sunday. In recent months, the militants destroyed two ancient temples in the central Syrian city.

The official, Maamoun Abdulkarim, said witnesses in Palmyra had confirmed that the monument, the Arch of Triumph, a jewel in the exquisite collection of ruins in the oasis city, had been blown up.