The opposition Syrian National Coalition on Sunday condemned the reported "collective execution" by rebels of soldier prisoners in the north and said it had created a commission of inquiry.
At least 150 Syrian regime forces died in fighting with rebels for control of Khan al-Assal, a key town in the northern Aleppo province, the Syrian Observatory for Human Rights said on Friday.
It said more than 50 of those killed were executed by rebels after Khan al-Assal fell on Monday, while the rest died fighting for the regime's last bastion in the west of Aleppo province.
"Videos show what appears to be the collective execution of a number of soldiers," the opposition statement said.
"The Coalition condemns this act, and announces the creation of a commission of inquiry, stressing the need to take proceedings against those whose implication in the crime is proven."
The Coalition said first indications show "the implication of armed groups not affiliated to the general command of the Free Syrian Army" backed by Arab and Western governments.
The mainstream rebel Ninth Division of the FSA claimed responsibility for taking Khan al-Assal in a video statement distributed on Monday.
But footage distributed on Friday by the Observatory said jihadists including the Al-Qaeda-linked Al-Nusra Front and Liwa Ansar al-Khilafa were behind the takeover.
"The Coalition and the FSA general command condemn any violation of the Geneva Convention, no matter who is behind it," the opposition statement said.
Amateur video filmed by rebels and distributed by the Observatory showed the bodies of dozens of regime forces in a building.
"Mass graves for Bashar (al-Assad's) army," says one unidentified fighter, referring to the Syrian president, as a cameraman walks through the complex filming the corpses.
"These are Assad's dogs," the unidentified cameraman adds.
On Saturday, Syrian Foreign Minister Walid Muallem sent letters to UN chief Ban Ki-moon and the UN High Commissioner for Human Rights about the executions.
State media said his letter focused on "the horrible massacre committed by the terrorist group called Liwa Ansar al-Khilafa against dozens of civilians and soldiers".
Syrian Information Minister Omran al-Zohbi told state television late on Saturday that "the terrorists who committed a massacre at Khan al-Assal and the states that support and finance them will pay dearly for this crime".
Assad's regime refers to the insurgents trying to topple it as "terrorists".
Rebels had for months tried to take Khan al-Assal, a strategic town where 200 rebels and government forces were killed in fierce fighting over eight days in March.
Both sides have also traded accusations that chemical weapons in Khan al-Assal killed around 30 people, according to a March toll released by the Observatory and the regime.