The Al-Khalidiye and Al Zahraa neighborhoods, as well as Nile Street, were targeted by rocket fire on Saturday evening, Syria’s state SANA news agency reported. The munitions used by the rebels were rigged with toxic gas, causing dozens of civilians to suffer from asphyxiation, the agency said.

The hazardous substance has been preliminarily identified as chlorine, according to medical officials.

The head of the health department of the Syrian city of Aleppo, Ziad Haj Taha, reported that 50 people were taken to two Aleppo hospitals after the shelling, noting that the number of the injured is likely to rise.

“Ambulance services continue to provide assistance to victims of poison gas use by terrorist groups, presumably chlorine,” Taha told Russian Sputnik news agency.

Chlorine, which was first used as a weapon during WWI, may cause lasting health damage and can be fatal at high levels of exposure.

Children and women are reported to be among the victims of the attack.

The Syrian armed forces fired back, targeting the positions of the militants north of Aleppo, a SANA correspondent reported, adding that they appear to have suffered heavy losses.

“The Syrian army responded to the shelling of militants, attacking the positions from which the shelling was carried out, militants suffered significant losses,” a Syrian military source confirmed the retaliatory attack to Sputnik.

SANA has released a video showing purported victims of the attack wearing oxygen masks as they are being treated in an Aleppo hospital.

Moscow has repeatedly accused anti-government rebels of plotting a chemical weapons attack with the possible goal of framing Damascus. Last month, the Russian Defense Ministry reported that a hardline Islamist group affiliated with the Islamic State (IS, formerly ISIS/ISIL) got hold of two canisters filled with chlorine after raiding the headquarters of rival Jabhat al-Nusra group [rebranded as Hayat Tahrir al-Sham].