A report by the group warned torture and extra-judicial or summary executions of detainees in the context of an armed conflict are war crimes, and may constitute crimes against humanity if they are widespread and systematic.
HRW documented more than a dozen extrajudicial and summary executions by opposition forces.
It said three opposition leaders who were confronted with evidence of extrajudicial killings claimed only the “worst criminals were being executed”.
Meanwhile, an independent UN panel has confirmed an increasing number of “foreign elements” are now operating in Syria.
The investigative panel appointed by the Human Rights Council said some of these forces are joining armed anti-government groups, while others are operating on their own.
Activists say 23,000 people have been killed in Syria in the past 18 months.
Source: The Herald Scotland