BEIRUT, Lebanon — As rebel negotiators unveiled a new plan on Wednesday for a political transition in Syria, doctors in the city of Aleppo were still treating people in intensive care after an attack believed to involve the use of chlorine gas sickened more than 120 people, including 10 women and 37 children.

At least two people died in the attack, which witnesses said was carried out by government forces in the rebel-held section of Aleppo. Rescuers and citizen journalists who went to the scene said by text message that there had been a strong smell of bleach.

One of the victims, a 13-year-old girl named Hajer Kyali, died Wednesday afternoon. She had been in intensive care since the attack, which doctors said they believed had struck her family’s house directly, delivering a deadly dose of the gas.

Medical staff members described seeing people with symptoms such as shortness of breath, coughing, sneezing, irritation of the eyes, nausea and in some cases respiratory failure. Such symptoms are consistent with attacks involving chlorine, which can kill in high concentrations.