A Western intelligence agency revealed the accusations to the BBC a year ago, and claim it is in violation of an agreement by President Bashar al-Assad to destroy the nation's chemical munitions following a 2013 Sarin attack on Damascus.
The official said no U.S. troops had been killed or wounded in the attack, which was carried out by about 500 fighters, but one Syrian fighter allied with the coalition was wounded.
US Ambassador to the United Nations Nikki Haley said there was "obvious evidence from dozens of victims" of breathing difficulties to corroborate chlorine attacks in rebel-held Eastern Ghouta and Idlib province.
Idlib and Eastern Ghouta are two of four "de-escalation zones" which had been agreed upon by both sides of the conflict at peace talks in Kazakhstan a year ago.
The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, says Tuesday marked the "largest massacre in Syria" since April's chemical attack on Khan Sheikhoun in Idlib province when more than 80 people were killed. "Eastern Ghouta is experiencing a Middle Ages-style siege".
At least 300,000 people have been displaced in Idlib since mid-December, when the most recent regime push on the area began.
"What is happening in Eastern Ghouta is not simply a humanitarian crisis because aid is denied, these sieges involve the global crimes of indiscriminate bombardment and deliberate starvation of the civilian population", stated Mr. Pinheiro, Chairman of the Independent worldwide Commission of Inquiry on the Syrian Arab Republic. It said it ongoing fighting and the deliberate obstructions by forces at the front lines were blocking aid organizations from reaching civilians in need.
A USA military official said the US-led coalition that still assists Kurdish-led forces in the hunt for surviving IS members in eastern Syria killed at least 100 pro-regime fighters overnight.
The Syrian Civil Defence search-and-rescue group, known as the White Helmets, which works in opposition areas, called it "another bloody day for civilians" in eastern Ghouta.
The conflict in Syria is many-sided, with complex battle lines between multiple rebel factions, Islamist groups, government-backed troops and US and Russian forces.
Amid the attacks, doctors have put up makeshift health centres, but health professionals say they too are being hit.
The airstrikes on civilian residential areas killed 19 in Arbin, 17 in Jisrin, 2 in Zamalka, 3 in Sabka, 3 in Misraba, 7 in Hamuriya, 1 in Bayt Sava and another in Madyara neighborhoods, the sources told agencies.
Since then, activists say Russian and Syrian government forces have stepped up their attacks.
Observers say Russian Federation has ramped up its air campaign in Idlib after one of its fighter jets was shot down by rebels in the province on Saturday.
Rouhani was quoted as telling his Russian counterpart Vladimir Putin in a phone conversation that "tensions in northern Syria do not benefit anybody".