BAGHDAD — Islamic State militants have provoked a global outcry by attacking ancient monuments with jackhammers and bulldozers. But they also have been quietly selling off smaller antiquities from Iraq and Syria, earning millions of dollars in an increasingly organized pillaging of national treasures, according to officials and experts.

The Islamic State has defended its destruction of cultural artifacts by saying they are idolatrous and represent pre-Islamic cultures. Behind the scenes, though, the group’s looting has become so systematic that the Islamic State has incorporated the practice into the structure of its self-declared caliphate, granting licenses for digging in historic sites through a department of “precious resources.”

The growing trade reflects how the Islamic State fighters have entrenched themselves since seizing the Iraqi city of Mosul a year ago Wednesday, in a dramatic expansion of the territory they control in this country and neighboring Syria.

The extremist group’s recent capture of Syria’s majestic 2,000-year-old ruins at Palmyra threw a spotlight on the risk that the Islamic State poses to the region’s rich cultural heritage. It is, however, just one of 4,500 sites under the group’s control, according to the Paris-based Financial Action Task Force.

“They steal everything that they can sell, and what they can’t sell, they destroy,” said Qais Hussein Rasheed, Iraq’s deputy minister for antiquities and heritage.

Last June, militants seized the ancient Assyrian capital of Nineveh. In a video released this year, the Islamic State showed its fighters drilling off the faces of the mighty stone-winged bulls on the gates of the city. Militants also filmed themselves destroying statues at Mosul’s museum, but many items were actually replicas of antiquities kept in Baghdad, officials said.