Witnesses in Aleppo said the chlorine was apparently contained in a barrel bomb dropped on the residential neighbourhood of Sukkari on Tuesday afternoon. Aref al-Aref, a resident and activist, said he rushed to the area as soon as he heard the explosion and found people prostrate on the ground, without immediate evidence of injuries.

“There was no trace of shrapnel or gaping wounds or anything like that, which I thought was odd,” he said. “They were just coughing intensely and having trouble breathing, and there was this smell as if a swimming pool had exploded in the area.”

The White Helmets civil defence units said a total of 120 people were hospitalised and posted a video showing coughing people and children being administered oxygen at one of the local hospitals.

A statement from the al-Quds hospital, which received 46 of the patients, said that all were suffering from breathing difficulties and “a strong smell of chlorine emanated from their clothes.”

The use of chlorine as a weapon of war is banned under international conventions. Yet despite repeated appeals from the United Nations and other members of the international community, the Syrian government continues to use it on a regular basis, as a supplement to the other weapons it deploys in pursuit of its effort to crush the five-year-old rebellion against President Bashar Assad.

This was the second chlorine attack in a month in rebel-held Aleppo, and though chlorine attacks kill fewer people than the relentless conventional bombings that claim dozens of lives on a daily basis, they deepen the fears of people trapped by the war.

“I saw the horror of all the people. Everyone was scared,” said Abdulkafi Hamdo, an Aleppo activist who arrived in the area shortly after the attack. “They were shocked. They don’t know what to do. ‘It’s chlorine,’ they were saying. ‘What will they use after this?’ ”

The US-Russian negotiations have focused on securing a cease-fire around Aleppo and the delivery of humanitarian aid to civilians, along the route that was captured by the government on Sunday.

US officials have said they plan to keep talking to Russia and are still hopeful they can secure a deal. But now that the Syrian government has succeeded in surrounding Aleppo entirely, it is unclear whether the forces loyal to Assad would be prepared to accept terms that would impede their ability to continue to attack the rebels, even if the United States and Russia were to agree.

Also on Tuesday, the Turkish government said that two Turkish soldiers were killed and five were injured in an attack by the Islamic State on two Turkish tanks in northern Syria. They were the first casualties inflicted by the Islamic State since Turkey dispatched troops and tanks to the area.