Meanwhile, Turkey dispatched a large military convoy deep inside rebel-held territory to establish an observation post at Tel al-Eiss, close to the front line with government forces, according to activists who filmed the convoy's movements.
The SOHR said Tuesday's death toll was the highest recorded in Eastern Ghouta in more than a month.
In Geneva, the UN-mandated "Independent International Commission of Inquiry on the Syrian Arab Republic" issued a statement on Tuesday, condemning the "scale and ferocity" of regime attacks in Idlib governorate and eastern Ghouta, including the bombing of three hospitals.
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.
But the Americans have refused, creating the potential for an unprecedented armed conflict between two North Atlantic Treaty Organisation allies, the United States and Turkey - the latest twist on the seven-year-old war in Syria.
This is not the first time the Syrian government is accused of using chemical attacks that have killed civilians.
On Wednesday, France's foreign minister said "all indications show us today that the Syrian regime is using chlorine gas at the moment".
On Monday, the opposition Syrian National Coalition condemned what it described as a "barbaric onslaught" by Russian and Syrian forces in Idlib province in recent days and demanded United Nations action.
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Observatory chief Rami Abdel Rahman added that few rebels were among the dead because they rarely left their tunnels and had better protection from air strikes than civilians, estimated to number around 400,000 in Ghouta.
Among them was the young daughter of Abu Raad, a shell-shocked man from Eastern Ghouta.
Paulo Pinheiro, in a statement, added that the spiraling violence in Syria had made "a mockery of the so-called "de-escalation zones" - an agreement previous year between Russia, Iran, and Turkey to stabilize the lines of conflict and open corridors for urgently needed humanitarian relief.
The Syrian Civil Defense search-and-rescue group, known as the White Helmets, which works in opposition areas, called it "another bloody day for civilians" in Eastern Ghouta.
"We implore the worldwide community to speak with one voice, taking every opportunity to publicly pressure the Assad regime, and its supporters, to cease its use of chemical weapons", spokeswoman Heather Nauert said in a statement Monday night.
Those strikes followed an onslaught of airstrikes by both Russian and Syrian warplanes against rebel-held towns and villages in Idlib on Sunday and Monday, in apparent retaliation for the downing of a Russian warplane in the province Saturday.
The process to provide people with United Nations humanitarian aid was resumed a few days ago in areas on the Syrian border with Turkey. "If anything, these have been serious escalation areas", he was quoted as saying by the Guardian.
"Many civilians trying to return home have been killed and injured by unexploded ordinance", it stated.