Story highlights The mother allegedly pleads with her son to flee the ISIS capital of Raqqa

He turns her in to ISIS authorities for apostasy

The son executes his own mother publicly as hundreds look on, two Syrian rights groups report

(CNN) An ISIS fighter has executed his own mother before a public audience, an expat Syrian rights group said.

The 20-year-old killed his mother in the Syrian city of Raqqa, ISIS' de facto capital, as hundreds looked on near the post office where she worked, said the Syrian Observatory for Human Rights.

ISIS had accused her of apostasy after her son turned her in, the activists said. She allegedly had been "inciting her son to leave the Islamic State." She wanted to escape with him and told him "that the coalition will kill all members of the organization."

The observatory reported that the victim was in her 40s. The activist group Raqqa is Being Slaughtered Silently reported that she was 35.

CNN has not been able to independently confirm the reports of the rights groups.