A coalition of rebels including al-Qaeda’s-Nusra Front and the Free Syrian Army are continuing their offensive against the government-held western half of Aleppo. The UN harshly criticized them for their use of “indiscriminate” weapons, warning it could amount to war crimes.

Perhaps the biggest issue, though as-yet-unproven, is reports that the rebels have begun to use chlorine-filled shells in their attacks. Syrian media reported 35 civilians were suffering from effects of the toxic gas, though the rebels denied using them.

The Syrian Observatory for Human Rights reported at least 21 people killed over the weekend, amid heavy fighting around western Aleppo. The offensive began Friday, with the Nusra-led rebels trying to end a siege of their eastern half of the city.

This has been a recurring situation in Aleppo since it boiled down to just the military and this rebel faction, with both sides having repeatedly launched offensives and counter-offensives over the last several months, trading sieges and strikes against one anothers’ neighborhoods.

This has resulted in heavy casualties for the civilians stuck in the city, facing regular sieges that they are rarely as able to survive as the combatants, and taking the brunt of the rockets, airstrikes, and artillery attacks. The addition of chemical weapons, even primitive ones, threaten to make the matter far worse.