In a shocking low even by the standards of the so-called Islamic State, a militant has publicly executed his own mother after accusing her of apostasy.

The activist group Raqqa is Being Slaughtered Silently (RIBSS) said 20-year-old jihadi Ali Saqr al-Qasem shot his mother Lena, 45, in the head with an assault rifle in front of a large crowd.

Lena al-Qasem is understood to have been accused of apostasy – a crime that usually means leaving one’s religion but in practise is used by Isis as a justification for murdering anybody who doesn’t support or speaks out against the terror group.

The exact charge against Ms al-Qasem was “inciting her son to leave the Islamic State and escaping together to the outside of Raqqa”, according to the Syrian Observatory of Human Rights.

The UK-based conflict monitor said Ali Saqr al-Qasem had reported his mother to his Isis superiors, who then sentenced her to death and ordered him be the one to kill her.

The Observatory said hundreds of people turned out to watch Ms al-Qasem's execution.It is not known why her son was given the task of killing his own mother but the reason the execution took place outside Raqqa’s post office is because that is where Ms al-Qasem had worked.

The news comes as Isis’ chief spokesman in neighbouring Iraq, Abu Mohammed al-Adnani, was reportedly left with severe injuries following an airstrike. 

Al-Adnani, who has been singled out as a potential successor should anything happen to Isis’ leader Abu Bakr al-Baghdadi, required initial emergency treatment in the jihadi-held city of Hit after losing large amounts of blood, Iraq’s Joint Operations Command said.

He has since been moved to Isis’ Iraq-stronghold of Mosul, MailOnline reported, adding that his condition remains unknown.