Islamic State's self-declared police force in western Syria decapitated four men after accusing them of blasphemy, a rights group monitoring the Syrian conflict said on Saturday.

The men were beheaded in the countryside east of the city of Homs by the militant group's "Islamic Police", the British-based Syrian Observatory for Human Rights said.

The Observatory, which monitors the conflict using sources on the ground, reported a similar killing on Tuesday, when Islamic State beheaded a man in a town square in the north of the country.

Residents and activists say Islamic State has beheaded and stoned to death many people in areas it controls in Syria and Iraq for actions they see as violating their reading of Islamic law, such as adultery, homosexuality, stealing and blasphemy.

They have also killed rival fighters by similar methods off the battlefield and have set up patrols to police public behavior in their bid to establish a caliphate.

The Observatory also reported on Saturday that Islamic State had stoned a man and a woman to death for adultery in Manbij town in northern Syria after Friday prayers.

The group, which is the target of US-led air strikes in both countries, has also killed a smaller number of foreign captives.

The Observatory said last month that Islamic State had killed 1,432 Syrians off the battlefield since the end of June when it declared a caliphate in the territory under its control.