Unesco says the destruction of Palmyra's nearly 2,000 year old Baalshamin temple is a war crime.
The UN body said the destruction of the site by Islamic State militants was "an immense loss for the Syrian people and for humanity".
"The systematic destruction of cultural symbols embodying Syrian cultural diversity reveals the true intent of such attacks, which is to deprive the Syrian people of its knowledge, its identity and history," Unesco Director-General Irina Bokova said.
Ms Bokova said those responsible "must be accountable for their actions".
See more on the BBC here.