The United Nations says it has received reports that Syrian forces in Palmyra prevented civilians from leaving, ahead of its fall to Islamic State militants.
The UN, though not present in Palmyra, cited "credible sources".
It said it was "deeply concerned" about the plight of civilians remaining in Palmyra, amid reports of summary executions.
IS has also overrun the World Heritage site adjacent to the modern city, raising concerns about its future.
The militants have previously demolished ancient sites that pre-date Islam.
UN cultural organisation Unesco says its destruction would be "an enormous loss to humanity", but no damage has been reported there yet.