Who says? Putin's Russia, one of the murderous Assad regime's last remaining allies, that's who. Any new evidence? No. Any mention of the Assad regime's own intentional destruction of early Sunni religious sites or involvement of looting in places like Apamea? Of course not.
And why not? Could that be because the claim that looted antiquities are a top ISIS funding source will be used to justify an UN resolution that could result in any antiquities that are seized being repatriated back to Russia's ally, that same odious Assad regime?
The aim of any UN Resolution or UNESCO State Party action should be preservation not repatriation. And any means used should respect the due process rights so lacking in places like Syria and Russia for that matter.