The bombing was one of the bloodiest in the eastern Ghouta region for over a month.
Airstrikes and shelling of areas held by anti-government armed groups has compounded a three-year siege in which almost 400,000 civilians - including children - have had little access to basic assistance, including food, medicines and life-saving health assistance.
Streets in the stricken area were covered in all-enveloping dust as one strike after another hit the same neighborhoods.
Eastern Ghouta is the last rebel-held enclave near Damascus, and parts of it have been held virtually for the entire war. "No area at all in Ghouta is safe".
Hundreds of thousands are trying to escape the intensified assault and are seeking refuge at makeshift camps near Turkey's border. Opposition activists say dozens of people have been killed and at least three hospitals shut down in the past two days.
The region has been under intense attack since the end of December as the government of President Bashar Assad struggles to bring it under control. "Our air defense systems blocked them and destroyed majority".
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The Syrian American Medical Society said 11 people were treated for chlorine gas poisoning in hospitals in Idlib, and at least 28 civilians were killed during airstrikes that targeted areas near Damascus, the Syrian Observatory for Human Rights said, Bloomberg reported.
In the south, civilians in Al-Rukban camp remain inaccessible to the humanitarian team in Syria.
Syrians react near a badly injured person following regime strikes on the besieged rebel-held enclave of Kafr Batna near Damascus on February 6, 2018 in which 35 civilians were killed.
Free Syrian Army (FSA) rebels launch an attack on the capital Damascus from its suburb of Ghouta in July 2012, more than a year after the start of an uprising against the regime.
Eastern Ghouta, home to about 400,000 people, is also reeling under a government siege that has prevented aid from getting in.
Ragıp Soylu, Washington correspondent for Turkey's Daily Sabah outlet, said on Twitter shortly after the news broke that local sources were claiming no chemical attack had occurred, but that the FSA fighters had been exposed to some form of smoke from another source.
United Nations war crimes investigators said on Tuesday that they were studying multiple reports that chemical weapons have been used in rebel-held zones.
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"What is happening in eastern Ghouta is not simply a humanitarian crisis because aid is denied, these sieges involve the worldwide crimes of indiscriminate bombardment and deliberate starvation of the civilian population", stated Mr. Pinheiro.
Chlorine bombs, which burst on impact, have been used on countless occasions during the war in Syria.
The White Helmets said airstrikes targeting the village of Termala in Idlib on Tuesday killed three people.
The appeal coincides with the collapse in recent weeks of a year-old Russian effort to tamp down the violence through "de-escalation zones", which had helped contribute to a perception that the war in Syria finally was winding down.
The UN's International Commission of Inquiry on Syria condemned the attacks, which cause collateral damage among the very people the government of Syria swore to protect.
That deployment is separate from a Turkish military offensive that Ankara launched last month to rout USA -backed Syrian Kurdish fighters from the northern enclave of Afrin.
Turkey's military said a Turkish soldier was killed in an attack in Idlib, where the Turkish troops are establishing an observation post.
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