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.”

Human rights observers are concerned that both the scale and the brutality of the attacks could ramp up if the conflict drags on. “If Bashar [Assad] stays, you will see more of this,” said Rami Abdul Rahman, director of the Britain-based Syrian Observatory for Human Rights, which monitors casualties on both sides. “Next time maybe more than 100 people killed. By anybody, the government or the rebels.”

Leaders of the Free Syrian Army said they did not condone the apparent revenge killings in Aleppo. “The Free Syrian Army did not give orders or accept what happened,” said Aref Hammoud, a spokesman for the FSA. “Many people joined the revolution and do not know that such acts are not accepted and that we should be better than the regime and not use its methods.”

A U.N. official said Wednesday that government forces have begun to use jets to strike at targets in Aleppo and rebel forces have started using tanks, increasing the level of violence in the city. “Our observers are reporting exchanges of fire, shelling and explosions in addition to the use of helicopters, tanks, heavy machine guns and artillery shelling,” said Susan Ghosheh, a spokeswoman for the U.N. monitoring team in Syria.

In an interview published Wednesday in a Syrian military magazine, Assad called the fighting “a battle that the whole fate of our people and the past, present and future of our nation depends on.”

Assad has not made a public appearance since a July 18 bomb attack killed four top government officials, fueling speculation that he has either fled to the ancestral homeland of his Alawite sect on Syria’s western coast or left the country.

The rebels have recently claimed big gains in Aleppo: Fighters have reportedly taken over five police stations in the past two days and control roughly half the city, according to Hammoud.

Rebel forces also have recently received a shipment of surface-to-air missiles, according to NBC, the kind of hardware they’ve been requesting for months, which may allow them to shoot down government airplanes and helicopters and perhaps turn the conflict in their favor.

Ahmed Ramadan and Suzan Haidamous contributed to this report.