Of the more than 36,000 killed so far in Syria, about one-fourth are regime soldiers, according to the Observatory. The rest include civilians and rebel fighters, but the group does not offer a breakdown.
Daily casualties have been rising since early summer, when the regime began bombing densely populated areas from the air in an attempt to dislodge rebels and break a battlefield stalemate.
Karen Abu Zayd, a member of the U.N. panel documenting war crimes in Syria, said the regime is to blame for the bulk of the atrocities so far, but that rebel abuses are on the rise as the insurgents become better armed and as foreign fighters with radical agendas increasingly join their ranks.
"The balance is changing somewhat," she said in a phone interview, blaming in part the influx of foreign fighters not restrained by social ties that bind Syrians.
Abu Zayd said the panel, though unable to enter Syria for now, has evidence of "at least dozens, but probably hundreds" of war crimes, based on some 1,100 interviews. The group has already compiled two lists of suspected perpetrators and units for future prosecution, she said.
Many rebel groups operate independently, even if they nominally fall under the umbrella of the Free Syrian Army. In recent months, rebel groups have formed military councils to improve coordination, but the chaos of the war has allowed for considerable autonomy at the local level.
"The killing of unarmed soldiers shows how difficult it is to control the escalation of the conflict and establish a united armed opposition that abides by the same ground rules and norms in battle," said Anthony Skinner, an analyst at Maplecroft, a British risk analysis company.
Rebel commanders and Syrian opposition leaders have promised human rights groups that they would try to prevent abuses. However, New York-based Human Rights Watch said in a report in September that statements by some opposition leaders indicate they tolerate or condone extrajudicial killings.
Free Syrian Army commanders contacted by the AP on Friday said they were either unaware or had no accurate details about the latest video.
Ausama Monajed, a member of the Syrian National Council, the main opposition group in exile, called for the gunmen shown in the video to be tracked down and brought to justice.
He added, however, that atrocities committed by rebels are relatively rare compared to what he said was a "massive genocide by the regime."
Regime forces have launched indiscriminate attacks on residential neighborhoods with tank shells, mortar rounds and bombs dropped from warplanes, devastating large areas. In raids of rebel strongholds, Assad's forces have carried out summary executions, rights groups say.
Rebels have also targeted civilians, setting off car bombs near mosques, restaurants and government offices. Human Rights Watch said in September it collected evidence of the summary executions of more than a dozen people by rebels.
In August, a video showed several bloodied prisoners being led into a noisy outdoor crowd in the northern city of Aleppo and placed against a wall before gunmen shot them to death. That video sparked international condemnation, including a rare rebuke from the Obama administration.
The latest video emerged on the eve of a crucial opposition conference that is to begin Sunday in Qatar's capital of Doha. More than 400 delegates from the Syrian National Council and other opposition groups are expected to attend to choose a new leadership.
U.S. Secretary of State Hillary Rodham Clinton has called for a more unified and representative opposition, even suggesting the U.S. would handpick some of the candidates.
Clinton's comments reflected growing U.S. impatience with the Syrian opposition, which, in turn, has accused Washington of not having charted a clear path to bringing down Assad.
The Syrian National Council plans to elect new leaders during the four-day conference but is cool to a U.S. proposal to set up a much broader group and a transitional government, said Monajed, the SNC member who runs a think tank in Britain.
U.S. officials have said Washington is pushing for a greater role for the Free Syrian Army and representation of local coordinating committees and mayors of liberated cities in Syria.
Nuland said that it would be easier for the international community to deliver humanitarian assistance to civilians and non-lethal aid to the rebels once a broader, unified opposition leadership is in place.
Such a body could also help persuade Assad backers Russia and China "that change is necessary" and that Syria's opposition has a better plan for the country than the regime, she said.
Associated Press writer Matthew Lee in Washington contributed to this report.
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