BEIRUT — Syrian President Bashar al-Assad framed his battle with the opposition as a decisive showdown over the fate of the country Wednesday, as opposition groups said at least 50 people had been killed by government forces in a Damascus suburb.

Opposition groups said the Wednesday killings took place in a suburb called Jdeidat Artouz, where civilians came under heavy attack by machine guns and mortar fire from the Syrian military. The Local Coordination Committees, an activist network, posted what it said was a live video feed from the site in the early evening, showing what appeared to be at least 20 bodies wrapped in white shrouds lined up in a mass grave. A bulldozer scooped up mounds of dirt and covered the bodies in the video as groups of men watched.

The claim of a new mass killing at the hands of the government came as human rights groups condemned what appeared to be reprisal killings of regime loyalists. A video that circulated Tuesday purportedly showed the execution of government supporters by rebel fighters in Aleppo, Syria’s largest city, raising the specter of more atrocities by both sides.

With rebel fighters making recent gains in the nearly 17-month uprising against Assad, U.S. Secretary of State Hillary Rodham Clinton last week warned them against revenge killings. “We think it’s very important that the opposition fighters, as they get better organized and expand their presence more broadly, send a message that this is for the benefit of all Syrians, not for any group, not engage in any reprisals and retribution that could lead to even greater violence than currently is taking place,” she said.

In the graphic video posted Tuesday, dazed and bleeding men are led out of a building as rebels hoist their AK-47s and chant “Free Army forever! Be ready for us, Assad.” The men are lined up below a mural of Mickey Mouse in what appears to be the courtyard of a school. Within seconds, a barrage of gunfire breaks out and continues for a full minute, before the person filming the shooting, whose lens is occasionally blocked by a hand, gets in close for a shot of what appears to be four bodies piled in a mangled heap.

Those killed were purportedly senior members of the Berri clan, a family closely allied with the Syrian government and, according to Aleppo residents, linked to organized crime and drug smuggling.

In a separate video apparently filmed before the execution, a number of the captured Berri clansmen identify themselves, including Zeino, a senior figure within the group, and Mohammed Shaaban, reportedly a parliamentarian from Aleppo. Both Zeino and Shaaban appear to be among the group that was shot.

This is not the first time that the two sides have been accused of abuses. The Syrian military was accused of killing more than 100 civilians in the town of Houla in May and more than 70 civilians in Qubair in early June.

The opposition also has been accused of abuses, but not nearly on the scale as the Syrian government. Amnesty International issued a report last week that said its researchers had seen a handful of videos “depicting individuals being summarily killed by members of Syrian armed opposition groups.”