An Islamic State fighter in Syria shot and killed his mother after the extremist group told him she was a non-believer, activists said.

​According to the Britain-based Syrian Observatory for Human Rights, which tracks the Syrian conflict, the 20-year-old fighter killed his mother in front of a large gathering in the northern Syrian city of Raqqa, the de facto capital of the Islamic State's self-proclaimed caliphate.

Abu Mohammed, a member of a Raqqa-based activist group that reports on ISIS, known as Raqqa is Being Slaughtered Silently, also reported Thursday's killing on his Twitter account.

The Observatory said it took place near the local post office building, where the woman used to work.

ISIS has brutally killed scores of people opposed to its rule in the so-called caliphate it runs in areas under its control in Syria and Iraq.

18 ISIS fighters killed 

Clashes between a joint Turkish-Iraqi force and Islamic State militants near a training camp outside the northern Iraqi city of Mosul have left at least 18 ISIS fighters dead, officials said Friday.

The fighting erupted late on Thursday outside the Bashiqa camp, which was at the centre of a controversy last month when Turkey moved troops there to protect Turkish trainers aiding local Sunni fighters hoping to take back Mosul from the Islamic State group.

Baghdad has demanded that Turkish troops withdraw, describing their presence as a violation of international law. Ankara has pulled some troops out but not all.

Turkish President Tayyip Erdogan said on Friday that ISIS tried to infiltrate Bashiqa, triggering the clashes. Former Iraqi governor Atheel al-Nujaifi, who founded the training camp, said the attack was repelled.