HRW says hundreds affected by use of toxic chemicals contained in barrel bombs dropped over Idlib in March.
The UN Security Council has heard firsthand accounts from Syrian doctors of alleged chemical weapons attacks in their country.
Following the closed-door meeting at the UN headquarters in New York on Thursday, Samantha Power, US ambassador to the UN, said the Security Council will now seek to identify those behind the attacks and ensure they face justice.
She said council members became “very, very emotional” while watching a video of the doctors struggling to save children who were vomiting and choking.
“If there was a dry eye in the room, I didn’t see it,” Power said.
The US, along with Britain and France, accuse President Bashar al-Assad’s regime of using chlorine gas against civilians, but Russia maintains there is no firm evidence that Damascus is responsible.
“All the evidence shows that they come from helicopters – only the Assad regime has helicopters,” Power said.
The 15 envoys heard a report by a Syrian doctor who said that he last month treated victims of a chlorine bomb attack on the village of Sarmin, near Idlib in northern Syria, that killed six people, including three children.
Dr Zaher Sahloul, of the Syrian American Medical Society, told Al Jazeera that the scene was “very traumatic”.
“Doctors and nurses were struggling to treat them. They all had respiratory symptoms, they had coughs, they had wheezing, some of them had fluid in their lungs,” Sahloul said.
That attack took place on March 16 – just 10 days after a rare moment of agreement when the Security Council passed a resolution condemning the use of chlorine.
“Despite this apparent act of defiance by Damascus, any action by the UN Security Council looks almost impossible,” Al Jazeera’s Diplomatic Editor James Bays reported from the UN headquarters.
“Russia is likely to veto any further condemnation of the Assad government. The Obama administration, for all its strong words, remains deeply reluctant to get involved militarily in Syria.”
The global watchdog, the Organisation for the Prohibition of Chemical Weapons, is carrying out a fact-finding mission in Syria on the use of toxic chemicals during the attacks last month and their report will be key to guiding the council on next steps.
US-based Human Rights Watch has said it investigated six reported attacks in Idlib and villages outside the city, collecting evidence from rescue workers and civilians that provided a compelling case in three of them.
The Security Council met amid reports that the Syrian government has stepped up air strikes in the past two days over Aleppo and Idlib, killing at least 40 civilians.
The UK-based Syrian Observatory for Human Rights said on Thursday that at least 120 air aids were carried out, and dozens of barrel bombs were dropped targeting Idlib and rebel-held parts of Aleppo.
Al Jazeera could not independently verify the reports.