BEIRUT — The Syrian government dropped a bomb containing chlorine on a besieged neighborhood in the city of Aleppo on Tuesday, heightening fears among people who are cut off from the outside world and unable to escape, according to residents and hospitals in the area.
The attack came as government loyalists battled to consolidate their hold over what had been the last rebel supply line into the opposition-held east of the city, after the capture of the route on Sunday.
The outcome of the battle meant that eastern Aleppo is now completely besieged for a second time in two months, and coincided with the failure of talks between the United States and Russia for a cease-fire deal in the contested city.
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Witnesses in Aleppo said the chlorine was apparently contained in a barrel bomb dropped on the residential neighborhood 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 defense units said a total of 120 people were hospitalized 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.
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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 daily, 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 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.
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The Turkish fatalities came after Turkish troops and allied Syrian rebels on Sunday expelled the Islamic State from the last strip of territory the militant group controlled along the Syrian-Turkish border, effectively sealing the extremists’ self-styled caliphate off from the outside world.
Turkey launched the incursion into Syria — the so-called Euphrates Shield operation — to back Syrian rebels in their fight to push the Islamic State out of the town of Jarablus and to limit the Syrian Kurdish forces’ advance west of the Euphrates River.
In a statement, Turkey’s military said the militants fired rockets at Turkish tanks during clashes near the border area from where the Islamic State was pushed out of on Sunday. It said the wounded were evacuated by helicopters. Two Turkey-backed Syrian rebels were also killed and two wounded rebels were also evacuated.
The Britain-based Syrian Observatory for Human Rights said there were intense clashes on Tuesday between the Turkish-backed rebels and Islamic State militants east of the town of al-Rai and surrounding villages, the Associated Press reported.
The territorial losses at the border were the biggest blow to the militant group, which also has suffered a series of recent battlefield setbacks elsewhere in Syria and in neighboring Iraq.
The two killed by the Islamic State were not Turkey’s first casualties after the launch of the incursion, though they were the first fatalities at the hands of the militant group since the operation began.
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On the fifth day of the operation, a Turkish soldier was killed in clashes with Kurdish fighters in northern Syria. In July 2015, a Turkish soldier was killed after Islamic State militants shot across the border into Turkey. Ankara conducted airstrikes against the Islamic State inside Syria after that.