A May 13 report from Amnesty International notes claims that chemical weapons were used by Syrian rebels against the besieged Kurdish enclave of Sheikh Maqsood in the divided city of Aleppo. Factions in the rebel alliance known as Aleppo Conquest "have repeatedly carried out indiscriminate attacks—possibly including with chemical weapons—that have struck civilian homes, markets and mosques, killing and injuring civilians, and have displayed a shameful disregard for human life," Amnesty said. It noted that two of these factions, Army of Islam and Ahrar al-Sham, have sent representatives to the UN-brokered negotiations in Geneva, while the others have approved delegates to represent them at the talks.

There are around 30,000 civilians living in Sheikh Maqsood, a district controlled by the Kurdish-led People's Protection Units (YPG) forces, and the area has come under sustained attack from rebel groups that control areas to the north, east and west.

Among the weapons used by the armed groups are unguided projectiles that cannot be accurately aimed at specific targets, including improvised "Hamim" rockets and home-made projectiles fitted with gas canisters known as "hell cannons."

Amnesty has obtained the names of at least 83 civilians, including 30 children, who were killed by attacks in Sheikh Maqsood between February and April. More than 700 civilians were also injured, according to the local field hospital. Video evidence seen by Amnesty shows artillery shelling, and rocket and mortar attacks carried out by Aleppo Conquest (Fatah Halab) on the district. Satellite imagery obtained by Amnesty and corroborated by residents shows destroyed and badly-damaged houses in residential streets in Sheikh Maqsood, more than 800 meters away from the frontline.

One doctor in Sheikh Maqsood told Amnesty that on April 7-8 he treated six civilians and two YPG fighters for symptoms including shortness of breath, numbness, red eyes and severe coughing fits. He said that several of the victims reported seeing yellow smoke as missiles impacted. A toxicologist consulted by Amnesty, who viewed video clips of the apparent attack and reviewed the doctor's testimony, said the symptoms could be the effects of  chlorine gas. A subsequent statement purportedly issued by the Army of Islam said a field commander had deployed an "unauthorized weapon" on Sheikh Maqsoud and would be held to account.

Amnesty International’s Middle East deputy director Magdalena Mughrabi said the Fatah Halab coalition "has launched what appear to be repeated indiscriminate attacks that may amount to war crimes. By firing imprecise explosive weapons into civilian neighborhoods the armed groups attacking Sheikh Maqsoud are flagrantly flouting the principle of distinction between civilian and military targets, a cardinal rule of international humanitarian law. 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."

Other groups named as part of the Fatah Halab coalition include al-Shamia Front, the Sultan Murad Brigade, the Sultan Fatih Battalions, the Fa Istaqim Kama Omirt Battalions, the Nour al-Deen Zinki Battalions, the 1st Regiment (al-Foj al-Awal) and the Abu Omara Battalions.

According to the Syrian Network for Human Rights, at least 23 civilians were meanwhile killed by YPG shelling and sniper attacks in rebel-held areas of Aleppo between February and April.

Amnesty is calling on the Gulf states, Turkey and others believed to be supporting armed groups in Syria "to immediately block the transfer of arms to such groups, including logistical and financial support for such transfers, where there is credible evidence that they have committed serious human rights abuses and violations of international humanitarian law."