“Every person on the planet should pause after yesterday’s violent attack on humanity’s heritage and understand ISIS’ intent not only to control the future of humankind but also to erase and rewrite our past,” said Deborah M. Lehr, chairwoman and co-founder of the Antiquities Coalition, a Washington-based archaeological advocacy group.

“We must unite with global intention to preserve our common heritage and resist ISIS’ effort to steal not only our future freedom but also our history, the very roots of our civilization,” she said in a statement on its website.

The Nimrud destruction came a week after Islamic State militants videotaped themselves marauding through Mosul’s museum, using sledgehammers and torches to destroy statues, artifacts and books. “They’re taking us back to the dark ages, those people,” said Mohamed Alhakim, Iraq’s ambassador to the United Nations. “They are thugs.”

Ms. Bokova, who was visiting the United Nations headquarters in New York on Friday to attend a Security Council meeting over the plundering of artifacts in northern Iraq, said in an interview that “protecting cultural heritage is not a luxury, it’s an imperative.”

Asserting that she had not been taken seriously over worries about cultural looting and destruction at the start of the Syrian conflict four years ago, Ms. Bokova expressed hope that governments around the world, spurred by a Security Council resolution passed nearly four months ago, would now strengthen customs officers and courts to crack down on pilfered antiquities.

Ms. Bokova said Unesco had been working with auction houses, Interpol, and officials from several countries to track the trade in stolen objects.