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A number of prominent human rights groupsÌýappealed to the United Nations on Monday to open an investigation into the massacre.

TheÌýgroups urgedÌý, US envoy to the UN,Ìýto look into theÌý, in whichÌý41 civilians were killed by Syrian regime forces in the Tadamon neighbourhood of Damascus in April 2013.

"We are writing to demand immediate action to address this massacre, which amounts to a war crime, and hold perpetrators accountable at the UN Security Council," the letter, issued by the (SNHR), read.

The SNHR also appealed to the US to hold a meeting at the during its presidency in May and launch an independent inquiry into the case.

The appeal comes two weeks after theÌýGuardian newspaper published a report that included leaked video footage purportedly showing evidence ofÌý committed against civilians by Syrian forces.

The footage showed blindfolded and handcuffed civilians being instructed to run towards an execution pit located in front of them in the Tadamon district. The footage also showsÌýmembers of the infamous Branch 227 appearing amusedÌýas they the civilians, after which theyÌýdousedÌýgasoline on the bodies and set them alight.

"Never before have we seen such clear evidence of a war crime committed and videotaped by Assad’s intelligence services in broad daylight, , with no regard for the humanity of the victims or concern for consequences," the SNHR continued in the letter.

The video triggered a public outcry from Syrians, with some families recognising relatives being executed in the footage.

The civil society groups have also decried the lack of response from the international community, particularly in the context of committed by Russian forces in Ukraine.

"It's high time that the international community learned that impunity for grave human rights violations in Syria has far-reaching consequences beyond its borders,"Ìýthey concluded.

Rights groups estimate that around 500,000 people have in the Syrian conflictÌýsince its onset in 2011. Advocacy groups also stateÌýthat scores of Syrians remain displaced and missing, including children.