The video raises concerns over the rebels just ahead of a major conference this weekend in Qatar at which the United States is trying to unify the opposition under a new leadership. Washington and its allies have been hesitant to give stronger support to the rebellion in part because of worries over its multiple divisions and lack of organization.
The killings took place Thursday during an assault by rebels on the northern town of Saraqeb, the scene of heavy fighting in past weeks between rebels and forces of President Bashar Assad's regime, according to an anti-regime activist organization, the Syrian Observatory for Human Rights.
Rebels are now in full control of Saraqeb after regime troops pulled back during Thursday's fighting, the Observatory said. That gives the rebels a strategic point on the main highway linking Syria's largest city Aleppo - which rebels have been trying to capture for months - with the regime stronghold of Latakia on the Mediterranean coast.
Human rights groups said they were trying to confirm the video's authenticity. The footage was consistent with other Associated Press reporting in the area. The video is dated Thursday, a day when the Observatory reported heavy attacks by rebels on regime checkpoints at Saraqeb.
The amateur video, posted on YouTube, shows a crowd of gunmen, apparently rebels, in what appears to be a building under construction.
They surround a group of captured men on the ground, some on their bellies as if ordered to lay down, others sprawled as if wounded. Some of the men are in Syrian military uniforms. "These are Assad's dogs," one of the gunmen is heard saying of the captured men.
The gunmen kick and beat some of the men, who appear terrified as one gunman shouts, "Damn you." The exact number of soldiers in the video is not clear, but there appear to be around 10 of them.
Seconds later, amid the screams of those captured, gunfire erupts for around 35 seconds and the men on the floor are seen shaking and twitching, apparently from being struck by bullets. The spray of bullets raises a cloud of dust from the ground.
The video is titled "prisoners and dead from the regime military at the Hmeisho checkpoint." On Thursday, the Observatory had reported 12 soldiers killed at Hmeisho, outside Saraqeb, one of three major rebel attacks on military checkpoints in the area.
Thursday saw heavy casualties for the military around the country, with 83 soldiers killed in attacks by rebels and clashes, the Observatory said. Half of those were killed in Idlib province, where Saraqeb is located.
There have been multiple allegations of massacres by both rebels and government troops during Syria's 19-month-old uprising, which has plunged into outright civil war. At least 36,000 people have been killed since it began in March 2011, according to anti-regime activists. Thousands of people have been killed over the past few months, including more than 500 last week during a four-day internationally-brokered cease-fire that collapsed shortly after it went into effect.
On Wednesday, the Obama administration said it would push for a major shakeup in the opposition leadership so that it better represents the fighters risking their lives on the frontlines. The opposition's political leadership, mostly in exile, has been criticized as increasingly irrelevant and out of touch.
U.S. Secretary of State Hillary Rodham Clinton said the administration was suggesting names and organizations that should feature prominently in any new rebel leadership that is to emerge from a four-day conference starting Sunday in Doha, the capital of Qatar.
London-based Amnesty International and the U.N. High Commissioner for Human Rights both said they were trying to confirm the video's authenticity and determine the identity of the gunmen.
"This shocking footage depicts a potential war crime in progress, and demonstrates an utter disregard for international humanitarian law by the armed group in question," said Ann Harrison, Amnesty's deputy Middle East and North Africa program director.
UNHCHR spokesman, Rupert Colville, said "the allegations are that these were soldiers who were no longer combatants and therefore, at this point, it looks very like a war crime."
"Unfortunately this could be the latest in a string of documented summary executions by opposition factions as well as by government forces and groups affiliated with them, such as the (pro-government) shabiha" militia, he told reporters in Geneva.
"The people committing these crimes should be under no illusion that they will escape accountability, because there is a lot of accumulated evidence, perhaps including this video," he said.
Rami Abdul-Rahman, who heads the London-based Observatory, asked how rebels can demand rights at a time when they violate such rights.
Fadi al-Yassin, an activist in Idlib province that includes Saraqeb, told The Associated Press that rebels captured three checkpoints around the town on Thursday after heavy fighting. Al-Yassin said he did not see the video.
"Some soldiers surrendered, while others fought," he said. "It could be in individual act. There are rebels who lost loved ones or suffered at the hands of the regime."
Al-Yassin said captured soldiers are usually well-treated by rebels who get information from them then refer them to makeshift tribunals. Those who are innocent are set free, he said.
As rebels gain more territory and a multitude of militias, jihadi Islamic militants and criminals join the fight against Assad, reports of serious human rights abuses committed by armed opposition elements are on the rise.
Previous videos of rebels executing soldiers and pro-Assad militiamen have fueled concerns that opposition fighters are capable of brutality that matches that of the regime they are seeking to topple - a charge that could badly damage the rebellion's ability to claim the moral high ground in the Syrian civil war.
Also Friday state TV reported that two bombs went off minutes apart in the Damascus neighborhood of Zahira al-Jadida wounding 16 people.
(Copyright 2012 by The Associated Press. All Rights Reserved.)