Rebels fired a flurry of shells against the northern Syria city of Aleppo on Saturday, wounding dozens of people. Scores were hospitalized, according to the Syrian Observatory for Human Rights, with breathing problems.

Syrian state media reported that the shells contained chlorine gas, and that over 100 people were wounded overall. The rebels are denying that they used chemical in any way.

It is unclear which rebel faction fired the shells at the city. A handful of Islamist factions are known to have worked on limited chemical weapons capabilities. While such attacks cause panic and often sicken many, the rebels are rarely able to launch effective enough attacks to commit mass casualties.

State media said the shells hit two districts. Witnesses reported that the al-Razi hospital was treating those wounded in strikes or sickened by the chlorine gas. Though international officials have been deeply critical when the government’s forces are accused of chemical attacks, they’ve rarely said anything when the rebels are behind such attacks. It remains to be seen if that will hold true this time.