Rebel forces in Syria killed as many as 190 civilians and seized more than 200 hostages during a military offensive in August, Human Rights Watch says.

A report by the US-based group says the deaths occurred in villages inhabited predominantly by members of President Bashar al-Assad's minority Alawite sect near the coastal city of Latakia. It said the findings "strongly suggest" crimes against humanity were committed.

The group has previously documented atrocities by pro-government forces. The report calls on the UN to impose an arms embargo on all groups where there is credible evidence of their involvement in war crimes.

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