A suspected chlorine gas attack on an opposition-held neighbourhood in the Syrian city of Aleppo has caused at least one death and dozens of cases of suffocation, according to local activists and medical sources.

A video obtained by Al Jazeera shows what activists say is the aftermath of an attack in the rebel-held al-Sukkari neighbourhood in eastern Aleppo.

They say that the Syrian government used a helicopter to drop two barrel bombs loaded with gas on residents, killing at least one person and injuring more than 100.

Rescue workers also said that Syrian government helicopters dropped suspected chlorine bombs on the neighbourhood on Tuesday.

The Syrian Civil Defence and the Syrian American Medical Society posted videos and photos on social media showing children doused in water using oxygen masks to breathe.

Another reported use of chlorine gas in besieged eastern #Aleppo City. Casualties include many children. pic.twitter.com/HTQ2jcL0kJ — SAMS (@sams_usa) September 6, 2016

The UK-based Syrian Observatory for Human Rights, which tracks daily developments in the Syrian war, said more than 70 people in Sukkari were left choking and needed treatment after the dropping of barrel bombs by Syrian government helicopters.

"Activists on the ground are saying that victims that were rushed to the hospitals are experiencing breathing difficulties," Al Jazeera's Hashem Ahelbarra, reporting from the Turkish city of Gaziantep near the Syria-Turkey border, said.

"They say that symptoms are the same that they have experienced in the past and this led them to believe that this is a chlorine gas attack."

The opposition Aleppo Media Centre also charged on its Twitter account that Sukkari was the target of a chlorine attack.

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Last month, an inquiry by the UN and the Organisation for the Prohibition of Chemical Weapons found that Syrian government forces were responsible for two toxic gas attacks in 2014 and 2015 involving the use of chlorine.

The Syrian Civil Defence accused the government of another chlorine attack in August. The UN said they were investigating this allegation.

"Accusations by the opposition that the Syrian government dropped chlorine gas in a barrel bomb on Aleppo are likely to further increase tension in the city that is now besieged by government troops," Al Jazeera's Ahelbarra said.

"The UN said that they have been investigating reports of what they believed to be chlorine gas dropped on Aleppo; they say if those accusations are confirmed that would amount to war crimes."

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Meanwhile, rebel factions launched a new offensive in Aleppo on Tuesday to regain some of the areas they have recently lost.

Government forces put eastern Aleppo under siege on Sunday for a second time since July after advancing against rebels on the city's outskirts.

The city has long been divided between government-held areas in the west and opposition-controlled neighbourhoods in the east.

Last month, opposition forces managed to break the crippling blockade on several districts of Aleppo.

"The rebel push to break the siege in Aleppo failed in the past because of divisions among the different factions operating in the area and the growing Russian involvement in the Syrian conflict," said Al Jazeera's Ahelbarra.

"Many are now calling for a merger of all rebel factions ... for the purpose of taking over the city of Aleppo."

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Aleppo has been one of the areas hardest hit by escalating violence in recent months after a partial truce brokered by the United States and Russia in February crumbled.

It is estimated that about 300,000 civilians are currently trapped in the city.