A video that appears to show a unit of Syrian rebels kicking terrified, captured soldiers and then executing them with machine guns raised concerns Friday about rebel brutality at a time when the United States is making its strongest push yet to forge an opposition movement it can work with.

U.N. officials and human rights groups believe President Bashar Assad's regime is responsible for the bulk of suspected war crimes in Syria's 19-month-old conflict, which began as a largely peaceful uprising but has transformed into a brutal civil war.

But investigators of human rights abuses say rebel atrocities are on the rise.

At this stage "there may not be anybody with entirely clean hands," Suzanne Nossel, head of the rights group Amnesty International, told The Associated Press.

The U.S. has called for a major leadership shakeup of Syria's political opposition during a crucial conference next week in Qatar. Washington and its allies have been reluctant to give stronger backing to the largely Turkey-based opposition, viewing it as ineffective, fractured and out of touch with fighters trying to topple Assad.

But the new video adds to growing concerns about those fighters and could complicate Washington's efforts to decide which of the myriad of opposition groups to support. The video can be seen at http://bit.ly/YxDcWE .

"We condemn human rights violations by any party," U.S. State Department spokeswoman Victoria Nuland said, commenting on the video. "Anyone committing atrocities should be held to account."

She said the Free Syrian Army has urged its fighters to adhere to a code of conduct it established in August, reflecting international rules of war.

The summary execution of the captured soldiers, purportedly shown in an amateur video, took place Thursday during a rebel assault on the strategic northern town of Saraqeb, said the Britain-based Syrian Observatory for Human Rights, an activist group.

It was unclear which rebel faction was involved, though the al-Qaida-inspired Jabhat al-Nusra was among those fighting in the area, the Observatory said.

The video, posted on YouTube, shows a crowd of gunmen 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 lie down, others sprawled as if wounded. Some of the captives are in Syrian military uniforms.

"These are Assad's dogs," one of the gunmen is heard saying of those cowering on the ground.

The gunmen kick and beat some of the men. One gunman shouts, "Damn you!" The exact number of soldiers in the video is not clear, but there appear to be about 10 of them.

Moments later, gunfire erupts for about 35 seconds, screams are heard and the men on the floor are seen shaking and twitching. The spray of bullets kicks up dust from the ground.

The video's title says it shows dead and captive soldiers at the Hmeisho checkpoint. The Observatory said 12 soldiers were killed Thursday at the checkpoint, one of three regime positions near Saraqeb attacked by the rebels in the area that day.

Amnesty International's forensics analysts did not detect signs of forgery in the video, according to Nossel. The group has not yet been able to confirm the location, date and the identity of those shown in the footage, she said.

After their assault Thursday, rebels took full control of Saraqeb, a strategic position 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.

On Friday, at least 143 people, including 48 government soldiers, were killed in gunbattles, regime shelling attacks on rebel-held areas and other violence, the Observatory said.

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.