An Islamist militant has reportedly executed his mother in public in Syria after she pleaded with him to leave the so-called Islamic State (IS).

The Syrian woman was killed by her 20-year-old son in front of "hundreds of citizens" near a post office in the IS stronghold of Raqqa where she worked, the Syrian Observatory for Human Rights (SOHR) said.

Activists working for the UK-based monitoring group said several sources had confirmed the shooting of the woman, who was in her 40s.

They said she was executed "under the pretext of 'inciting her son to leave the Islamic State and escaping together to the outside of (Raqqa) and that the Coalition will kill all members of the organisation'."

The group said her son was instructed to carry out the public execution after reporting his mother to IS.

Raqqa, which is considered the de facto capital of IS, has been repeatedly targeted in airstrikes by America and its allies.