Shortly after 11 o'clock on the morning of April 7, a pale yellowish cloud of smoke briefly flashed across Sheikh Maqsoud, a northern neighborhood of Aleppo.

After a few seconds, it was gone. Pro-Assad media, including Iran's PressTV, quickly hailed the cloud as evidence of a chemical weapons attack by rebel groups. Others have since denied the claim.

However, the incident in Syria's largest city is now being taken as an example of other abuses carried out by rebel groups, including forces affiliated with the Free Syrian Army (FSA), at a time when intra-Syrian talks on the country's future, as well as Bashar al-Assad's government, flounder in Europe.

According to a recent report by Amnesty International, Syrian rebel groups may have actually used chemical weapons in Sheikh Maqsoud in early April, one in a series of "indiscriminate attacks that may account to war crimes."

Abuses on both sides

The Syrian regime and its allies have committed multiple, egregious abuses since the outset of the Syrian uprising in 2011. However, Amnesty's Middle East and North Africa Deputy Director Magdalena Mughrabi warned that the international community "must not turn a blind eye to the mounting evidence of war crimes by armed opposition groups in Syria."

"The fact that the scale of war crimes by government forces is far greater is no excuse for tolerating serious violations by the opposition."

The rights group said it had obtained the names of 83 civilians, including 30 children, killed by homemade rockets and so-called hell cannons, DIY-made Howitzers that fire projectiles fitted with gas canisters. Seven people - five civilians and two Kurdish fighters - were also hospitalized between April 7-8 with symptoms "including shortness of breath, numbness, red eyes and severe coughing fits," Amnesty said.

This broadly follows several testimonies collected in another report, seen by DW, by Syrian activists both inside and outside the country following the flare-up in Sheikh Maqsoud.

Two doctors from a hospital in Sheikh Maqsoud described symptoms of mild asphyxiation, extreme coughing, convulsions, vomiting and difficulty in moving. Three of the four testimonies closely match up in terms of symptoms, the number of victims and the time of the attack.

The second report squarely places the blame on Jaish al-Islam, a powerful Islamist faction found in and around Aleppo, Damascus and other areas of Syria.

Chemical or not?

Jaish al-Islam's Mohamed Alloush, who represents rebels at the Syrian opposition's High Negotiations Committee, previously issued a statement denying the group has ever possessed or used chemical weapons, or other "prohibited weapons," in the area.

A purported statement by Jaish al-Islam's Twitter account shortly after the alleged chlorine attack, stated that one of its commanders had been withdrawn and reprimanded for ordering the use of a "prohibited weapon."

Some observers have pointed to this as evidence of an admission about the use of chemical weapons.

Others, like Kyle Orton, research fellow at London’s Henry Jackson Institute, a right-wing think tank, say the claims of chemical weapons are "not so convincing."

"Every one of the given symptoms and the 'yellow smoke' could be literally anything," he argued. "​​​Chlorine might well have been used​, but it might also be smoke grenades or just the normal smoke from artillery, which is a serious respiratory irritant."

The Syrian opposition’s High Negotiations Committee (HNC), which represents different opposition factions​ ​including Jaish al-Islam​ at intra-Syrian peace talks​, was unavailable for comment.

Fatah Halab, the rebel alliance in Sheikh Maqsoud that consists of nominally FSA factions as well as Islamist, Salafist groups like Ahrar al-Sham and Jaish al-Islam, meanwhile responded to the allegations in a statement in Arabic and English, pointing to its "full adherence to the international humanitarian law and its utmost endeavour to protect civilians in all areas [sic]." Fatah Halab would launch an investigation to ensure its fighters respect international law, the statement added.

Fragmented conflict

These sorts of disputes are as much a reflection of the difficulties of verifying and confirming information coming out of Syria, as they are of the deeply polarized, fragmented conflict that it has become. However, events in Sheikh Maqsoud have raised further questions around the intra-Syrian talks after a series of alleged war crimes and other atrocities carried out by rebel forces.

A rebel coalition reportedly led by Ahrar al-Sham and al-Qaeda's Syria affiliate, Jabhat al-Nusra, recently attacked an Alawite-majority village north of Homs, killing or abducting more than a dozen civilians and then circulating gruesome photos of the aftermath.

Michael Stephens, a research fellow at London's Royal United Services Institute (RUSI) focused on Syria's Kurdish regions, told DW that recent atrocities in Aleppo and other northern areas reflected a "natural pathway of escalation" given unprecedented tensions between Syrian rebel and Kurdish groups. Aleppo, and the abuses committed there by all parties to the conflict, reflected a "microcosm" of the wider Syrian conflict, he said.

"People are fighting for territory and political control in a way that they weren't two years ago. The stakes are just much higher than they were," Stephens explained. "I think people have just realized that the state just isn't going to exist in some of these areas ever again. And, therefore, it's up for grabs. The more they fight, the more the fighting escalates. The more they get backing from different players, the more the fighting escalates. Then you get one massacre, then you get a revenge massacre," Stephens said. "This is Syria."

Orton, meanwhile, admitted that recent atrocities "undoubtedly give even further advantages to the Assad regime and its Iranian and Russian backers in a Geneva process [Syrian peace talks - the ed.] that was already considerably tilted in their favor."

"They have done damage to the rebels' cause," he said.