Syrian government forces used chemical weapons in rebel-held areas of Aleppo during the final weeks of the battle to retake the key city, killing at least nine people and wounding hundreds more, according to Human Rights Watch (HRW).
In a report on Monday, the US-based rights group said it had verified eight chemical attacks during the offensive from November 17 to December 13, adding that four children were among the victims.
HRW said it had interviewed witnesses, collected photos and reviewed video footage to reach the conclusion that chlorine bombs were dropped from government helicopters during the operation.
Around 200 people were wounded by the toxic gases used on opposition-controlled areas of the northern city, according to HRW.
One of the deadliest bombings hit the neighbourhood of Sakhur on November 20, killing six members of the same family, including four children whose lifeless bodies were shown on a video taken by the Shabha press agency.
Aleppo, once Syria's bustling commercial hub, had been largely divided between a government-held west and a rebel-controlled east since 2012.
Syrian forces, backed by Russia, launched an offensive in November to seize east Aleppo, a key battleground in Syria's nearly six-year war.
The government of Bashar al-Assad announced on December 22 that it had taken full control of the city.
HRW said its report did not find proof of Russian involvement in the chemical attacks, but noted Moscow's key role in helping the government to retake eastern Aleppo.
The HRW report detailed attacks on a playground, clinics, residential streets and houses that left scores of people struggling to breathe, vomiting and unconscious.
The alleged attacks, which may have involved as many as three helicopters operating jointly, took place in areas where government forces were poised to advance, said the rights group.
"The pattern of the chlorine attacks shows that they were coordinated with the overall military strategy for retaking Aleppo, not the work of a few rogue elements," said Ole Solvang, HRW's deputy emergencies director.
Louis Charbonneau, the UN director at HRW, told Al Jazeera that the senior military officials who would have been overseeing the battle for Aleppo had to know chemical weapons were used.
"This is industrial strength. People get a burning in their throats, their eyes tear up. Their lungs fill with fluid. Your body simply will not let you bring in air. You can actually see the yellow-green gas as it is moving through," he said, explaining the effects of the gas.
The actual number of chemical attacks could be higher, said the group, adding that journalists, medical personnel and other credible sources had reported at least 12 attacks in that period.
Chlorine use as a weapon is banned under the Chemical Weapons Convention, which Syria joined in 2013 under pressure from Russia.
The use of chlorine bombs as an indiscriminate weapon could amount to war crimes.
HRW urged the UN Security Council to impose sanctions on senior leaders in the chain of Syrian command, but such a move would likely be vetoed by Russia.
France and Britain are pushing the Security Council to ban the sale of helicopters to Syria and impose the first UN sanctions against Syrian military leaders and entities tied to chemical weapons development.
A joint investigation by the UN and the Organization for the Prohibition of Chemical Weapons found that several units of the Syrian army had used toxic weapons against three villages in northern Syria in 2014 and 2015.
It was the first time an international probe blamed Assad's forces after years of denial from his government in Damascus.