An Islamic State militant killed his mother in a public square in the Syrian city of Raqa who begged him to leave the organisation, a monitor said Friday.

Ali Saqr, 20, had reported his mother, Lina, to ISIS authorities in Raqa "because she tried to persuade him to leave ISIS and flee the city," the Syrian Observatory for Human Rights said.

Authorities subsequently arrested the woman and accused her of apostasy, the monitoring group said.

On Wednesday, she was shot to death by her son "in front of hundreds of people close to the mail service building in Raqa city," the Observatory added.

Observatory head Rami Abdel Rahman said the woman, who was in her forties, was living in the nearby town of Tabaqa but worked in Raqa city.

The incident was widely condemned online by social media users.

Raqa is the de facto Syrian capital of ISIS's so-called "caliphate," the territories it controls in Syria and Iraq where it imposes its harsh interpretations of Islamic law.

Among the crimes that warrant a death sentence in ISIS territories are homosexuality, "exposing jihadist genitalia," adultery and intercourse with animals, according to the Observatory.

Other acts punishable by death range from blocking roads to "betraying Muslims" and working with anti-ISIS groups including the "crusader" US-led coalition.

Even capturing and torturing an anti-ISIS activist or fighter without proper authorisation from militants authorities could be met with a death sentence.

But in ISIS-held territories, using child soldiers and "owning slaves", which are typically sexually abused, are both legal.

And the extremist group has been accused of carrying out mass killings, torture, rape and sexual slavery.

A US-led coalition has been striking the militants in Syria and Iraq for over a year.

More than 260,000 people have been killed in Syria's war since it erupted in March 2011.