Against the backdrop of the Syrian government’s brutal crackdown, the tactics of the country’s armed opposition groups have received considerably less scrutiny.

But today Human Rights Watch (HRW) issued an open letter to leading Syrian opposition groups calling on them to cease politically motivated kidnappings, torture and executions of government-supported militia members, regime supporters and security forces.

One example cited by HRW is this graphic video below that was uploaded to YouTube on Feb. 4 and shows a man hanging from a tree, surrounded by armed fighters. According to commentary in the video, the man was part of a government militia who was captured and executed by members of the Free Syrian Army.

“The Syrian government’s brutal tactics cannot justify abuses by armed opposition groups,” said Sarah Leah Whitson, HRW’s Middle East director. “Opposition leaders should make it clear to their followers that they must not torture, kidnap, or execute under any circumstances.”

The letter acknowledged that it can be difficult for opposition groups to identify some of these perpetrators, who do “not necessarily belong to an organized command structure” and may at times even be the work of criminal gangs operating in the name of the opposition.

It also addresses an uncomfortable truth about the religious undertones of the violence, acknowledging that “some of the attacks targeting Shias and Alawites appear to be motivated by sectarianism.”

Today, 70 percent of career soldiers and the majority of Syria’s elite Republican Guard are member of the country’s Alawite minority. Many of the shabiha, or militia gangs loyal to the regime that have been involved in the crackdown, are also made up of Alawites.

The protests that broke out in Syria a year ago had been largely peaceful, the letter says, until September 2011, when a “growing number of military defectors and local residents” turned to arms, reportedly for self-defense. When the government began large-scale military attacks on protests hubs across the country in early February 2012, HRW notes, the intensity of the fighting increased.

Recent violence has also included mysterious bombings four times over the last three months on state security compounds in Damascus and Aleppo.  Over the weekend, two blasts rocked security facilities in Damascus, prompting questions about whether armed Syrian opposition could have been responsible.

The United Nations estimates that more than 8,000 people have been killed in Syria since the protests broke out.