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.