Ancient treasures pillaged from conflict zones in the Middle East are being offered for sale on Facebook, researchers say, including items that may have been looted by Islamic State militants.

Facebook groups advertising the items grew rapidly during the upheaval of the Arab Spring and the ensuing wars, which created unprecedented opportunities for traffickers, said Amr Al-Azm, a professor of Middle East history and anthropology at Shawnee State University in Ohio and a former antiquities official in Syria. He has monitored the trade for years along with his colleagues at the Athar Project, named for the Arabic word for antiquities.

At the same time, Dr. Al-Azm said, social media lowered the barriers to entry to the marketplace. Now there are at least 90 Facebook groups, most in Arabic, connected to the illegal trade in Middle Eastern antiquities, with tens of thousands of members, he said.

They often post items or inquiries in the group, then take the discussion into chat or WhatsApp messaging, making it difficult to track. Some users circulate requests for certain types of items, providing an incentive for traffickers to produce them, a scenario that Dr. Al-Azm called “loot to order.”

Others post detailed instructions for aspiring looters on how to locate archaeological sites and dig up treasures.

Items for sale include a bust purportedly taken from the ancient city of Palmyra, which was occupied for periods by Islamic State militants and endured heavy looting and damage.