The destruction of Palmyra's ancient temple of Baalshamin is a war crime, the UN's cultural agency has said.
Syrian officials and activists reported on Sunday that Islamic State (IS) militants had blown up the temple.
In a statement, Unesco said it was "an immense loss for the Syrian people and for humanity".
IS took control of Palmyra in May, sparking fears for the site, considered one of the ancient world's most important cultural centres.