(CBS/AP) BEIRUT - A video emerged Friday showing more than a dozen bloodied corpses in Syria, some of them piled on top of each other and in military uniforms, in what the government said was a "massacre" by rebels in the northern province of Aleppo.
The circumstances of the killings were not immediately clear. But in the video which could not be independently verified the narrator said the dead were members of the "shabiha," or pro-regime gunmen.
Syria's state-run news agency, SANA, said terrorist groups had killed and mutilated at least 25 people in Daret Azzeh, a rebel-held area in the Aleppo countryside.
The government refers to rebels as terrorists.
"The terrorist groups in Daret Azzeh committed a brutal massacre against the citizens, whom they had kidnapped earlier in the day," SANA said.
The report said at least 25 people were killed, but others were missing.
Although the city of Aleppo, Syria's largest, has been relatively quiet since the uprising against President Bashar Assad's regime began in March last year, towns and villages around it have witnessed intense clashes between troops and opposition fighters. Activists say more than 14,000 people have been killed since the uprising began.
Daret Azzeh has endured intense government shelling over the past two weeks as Assad's forces try to regain areas taken by rebels.
Also Friday, the Britain-based Syrian Observatory for Human Rights said four senior army officers have defected from the regime. The group provided a video purporting to show two brigadier generals and two colonels who declared they were joining the opposition.
The group said the defections came Thursday the same day a Syria fighter pilot flew his MiG-21 warplane to neighboring Jordan, where he was given asylum.
Thousands of soldiers have abandoned the regime, but most are low-level conscripts. The Free Syria Army, the loosely linked group of rebel forces, is made up largely of defectors.
A diplomat present at the U.N. Security Council briefing earlier this week by the head of the U.N. observer mission in Syria, Major General Robert Mood, told CBS News' Pamela Falk on Wednesday that Gen. Mood had been surprised at the efficacy of the rebel forces, and to see that the majority of those forces appeared to be Syrian military defectors - including some at senior levels.
Mood said the opposition had clearly become better at fighting over the course of the past 15 months of the uprising, according to the diplomat.
CBS News national security correspondent David Martin reports that the rebel fighters are becoming increasingly better equipped, thanks to weapons and money supplied by Arab countries hostile to Assad.
The opposition consists of roughly 100 different groups, and as Secretary of State Hillary Clinton told Charlie Rose the U.S. is trying to organize them into a more unified force.
"We're also working very hard to try to prop up and better organize the opposition. We've spent a lot of time on that. It's still a work in progress."
The U.S, is not providing weapons to the insurgents, but Martin reports the CIA has begun advising other countries about which opposition groups should receive arms and money.