BEIRUT, Lebanon — Islamic State militants have razed a fifth-century Roman Catholic monastery and blown up one of the best-preserved first-century temples in Palmyra, the ancient Syrian city that is one of the world’s most important archaeological sites, according to government officials and local activists.

And that was just this past week — in one Syrian province.

Much like the grinding slaughter of human beings, the ravaging of irreplaceable antiquities in Syria and Iraq has become something of a grim wartime routine. Yet the cumulative destruction of antiquities has reached staggering levels that represent an irreversible loss to world heritage and future scholarship, archaeological experts and antiquities officials say.

It has accelerated in recent months as the self-declared Islamic State has stepped up its deliberate demolition and looting, piling onto battle damage wreaked by government forces and other insurgents in Syria’s four-year civil war. That has brought antiquities lovers on all sides to a new level of despair.

“I feel very weak, very pessimistic,” Maamoun Abdulkarim, Syria’s director general of antiquities, said Monday in a phone interview from Damascus, adding that with his inability to protect Palmyra, “I became the saddest director general in the world.”