Members of terror group Islamic State (Isis) have stoned to death two men in Syria after alleging they were gay.

According to the Britain-based Syrian Observatory for Human Rights (SOHR), this is the first execution that the militants have carried out against homosexuals.

"IS today stoned to death a man that it said was gay," SOHR told news agency AFP, adding that the insurgents killed the 20-year-old boy after they found on his phone videos showing him "practicing indecent acts with males."

The incident occurred in the eastern province of Deir Ezzor, where few hours before an 18-year-old boy accused of being gay was also stoned to death by the insurgents.

Activists on social media said that the dead men were opponents of IS and that the group had used the allegation as a pretext to kill them.

The two executions follow the killing of two women, carried out by IS members in the besieged Iraqi city of Mosul.

According to a Kurdish media network, the two women had repented in one of IS's mosques to spare their lives.

However, an Islamic judge rejected their repentance and ordered the arrest of the two women in early November.

It is not yet clear why a death sentence was issued for the two women.

The UN also said that IS stoned to death several women accused of adultery in Syria in November.

The terror group often carries out public executions in the vast swathes of Syria and Iraq it controls, where the jurisdiction has been replaced by the IS version of Sharia law.

The insurgents have released a video, Blood of Jihad 2, in which they explain how they recruit and train children to become fighters.

The footage shows children, defined by the insurgents as "fighting cubs", engaging in fights and choreographed performances while holding what appear to be AK-47s.

Children are also seen practising how to disarm people and capture enemies.