The rebel Free Syrian Army pledged on Wednesday to punish atrocities amid outrage over a video showing the mutilation of a corpse, as the regime ruled out discussing President Bashar al-Assad's departure in negotiations.
Meanwhile, Syria's Internet was down for the second time in a week and rebels attacked the central prison in Aleppo, sparking fierce fighting with regime forces, a watchdog said.
The mainstream rebel Free Syrian Army issued a statement after a gory video emerged online of an alleged insurgent cutting out and apparently eating the organs of a regime soldier.
"Any act contrary to the values that the Syrian people have paid their blood and lost their homes (for) will not be tolerated; the abuser will be punished severely even if they are associated with the Free Syrian Army," said the FSA.
Field commanders had been instructed "to begin a prompt investigation into the matter in which the perpetrator will be brought to justice."
Investigations would also be held into whether the rebel in the video is a member of the FSA or not.
The man in the video, identified as Khalid al-Hamad, defended his actions in an interview with Time Magazine, saying he was driven to them by footage on the dead soldier's phone showing him "humiliating" a woman and her two daughters, all of them naked.
He expressed sectarian hatred of the Alawite community, the religious minority to which President Assad belongs.
His actions were also condemned by Syria's opposition National Coalition, as well as the United States and the United Nations.
The gruesome incident raised new fears about the potential for grisly sectarian violence in Syria.
On Tuesday, the Syrian Observatory for Human Rights watchdog reported Al-Nusra Front jihadists summarily executed three army officers in the northern city of Raqa.
Amid the turmoil on the ground, the international community pushed for talks on a political solution to the conflict.
US Secretary of State John Kerry urged Syria's president not to squander the opportunity for negotiations, insisting that "enormous plans are being laid."
But while Kerry said Russia had informed him Syria has already chosen envoys for the conference, Syrian officials insisted Assad's departure -- a key opposition demand -- was not on the table.
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"Syria will not accept any dictate and its friends will not accept it either," Deputy Foreign Minister Faisal Muqdad told state television.
Kerry and Russian counterpart Sergei Lavrov announced last week plans for a new conference, intended to build on a deal agreed in Geneva last year.
The deal called for a halt to violence now in its third year, and a transitional government, but made no call for Assad to leave.
On Wednesday, Lavrov called on Syria's opposition to support Moscow and Washington in their efforts to work towards convening the peace conference.
"It is important for all participants to express articulate support for the Russian-US initiative to implement the Geneva communique," Lavrov was quoted as saying by Russian news agencies.
"We should mobilise support for this conference. All outside participants in this situation, all Syrian sides, should be mobilised."
The opposition National Coalition is scheduled to meet in Istanbul on May 23 to discuss the Russia-US proposal.
On the home front, state news agency SANA said the Internet blackout was caused by a fault with a fibre-optic cable.
The outage was also reported by Google, which showed a drop-off in Internet service that started on Wednesday morning, and by Internet tracking firm Renesys.
Internet in the country has regularly been cut, most recently on May 7, with it being restored some 24 hours later.
On the ground, Syrian troops backed by tanks and warplanes fought to repel the attack on the central prison in Aleppo after rebels blew up its walls in suicide car bombings, said the Observatory.
Some 4,000 prisoners, including Islamists, are held in the prison on the outskirts of the northern city largely under rebel control, said Abdel Rahman.
An explosion near a checkpoint outside the army chief of staff offices in central Damascus, killing one person, an AFP journalist said.
The Observatory said at least 38 people were killed on Wednesday across Syria, adding to the more than 94,000 deaths it has tallied since the conflict flared in March 2011.