The rebel Free Syrian Army pledged 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.

Syria’s Internet, meanwhile, was down for the second time in a week while on the ground rebels launched an attack on the central prison in Aleppo, sparking fierce fighting with regime forces, a watchdog said.

The mainstream rebel Free Syrian Army (FSA) issued its statement after a gruesome video of an alleged rebel fighter cutting out and apparently eating the organs of a regime soldier emerged online.

“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,” the group said in a statement.

It said 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, it said.

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 naked women and her two daughters.

He expressed sectarian hatred of all members of the Alawite community, the religious minority to which Assad belongs. His actions were also condemned by Syria’s opposition National Coalition, as well as the US State Department and UN rights official.

The gruesome incident raised new fears about the potential for grisly sectarian violence in Syria.

The Britain-based Syrian Observatory for Human Rights reported on Tuesday that three captured army officers had been summarily executed in the northern city of Raqa by fighters of an Islamist group, Al-Nusra Front.

Amid the turmoil on the ground, the international community continued to push for talks on a political solution to the conflict.

US Secretary of State John Kerry urged Syria’s president not to squander the opportunity to come to the table for negotiations, insisting “enormous plans are being laid.”

But while Kerry said Russia had informed him that Syria has already chosen envoys for the conference, Syrian officials insisted that Assad’s departure - a key opposition demand - was not on the table.

“Syria will not accept any dictate and its friends will not accept it either,” deputy foreign minister Faisal Muqdad told Syrian state television late Tuesday.

Kerry and his Russian counterpart Sergei Lavrov announced plans for a new conference last week, intended to build on a deal agreed last year in Geneva.

The deal called for a halt to the violence that is now in its third year, and a transitional government, but made no call for Assad’s departure.

Lavrov on Wednesday called on the Syrian 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 in the Swedish town of Kiruna 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 on Wednesday fought to repel the attack on the central prison in Aleppo after rebels blew up its walls in suicide car bombings, the Observatory said.

Around 4,000 prisoners including Islamists and common law criminals are held in the prison on the outskirts of the northern city, which is largely under rebel control, said Observatory head Rami Abdel Rahman.

The Observatory said Tuesday that at least 94,000 people have been killed in the Syrian conflict, revising upwards a previous toll of 82,000.