"The Syrian government is using chlorine gas with impunity," the former US ambassador to Syria, Robert Ford, told a House panel, warning that other nations like North Korea were watching the international response carefully.
An international consensus against using chemical weapons "forged after World War I is steadily eroding," he warned.
Assad has denied Western accusations of being behind a series of chlorine gas barrel bombings from helicopters over the northwestern province of Idlib since March with as many as 45 reported attacks in recent months.
But doctor Mohamed Tennari vividly described the night of March 16, when, after hearing helicopters over his home town of Sarmin, a wave of explosive barrel bombs were dropped filling the air with "a bleach-like odor."
"Dozens of people experienced difficulty breathing, with their eyes and throats burning, and many began secreting from the mouth," he told the House foreign affairs committee, speaking through a translator.
Among the victims were three small children, Aisha, three, her sister Sara, two, and brother one-year-old Mohammad. They "were a sickly pale color when they arrived, a sign of severe lack of oxygen and chemical exposure," Tennari said.
The doctors were forced to treat them on the body of their grandmother, who succumbed to the deadly poison, as they had no free beds.
"As quickly as we worked, we could not save them," he said, adding the children's mother and father also died after a chlorine-gas bomb fell down their ventilation shaft. Their basement where they had tried to shelter "became a makeshift gas chamber."
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- 'Obscene' suffering -
Even doctors used to dealing with death said they have been shocked by the horrific methods allegedly used by the regime in the four-year Syrian civil war aimed at ousting veteran strongman Assad.
Doctor Annie Sparrow, from the Ichan school of Medicine at Mount Sinai, New York, described working on the ground in Syria, her voice cracking with emotion.
"I'm a doctor and I'm very familiar with death. But I never seen a more obscene way to kill children and never watched so many suffer in such an obscene manner," she said.
"Syrian children and Syrian civilians deserve protection and the United States can provide it."
The US administration has long ruled out setting up a no-fly zone over parts of Syria, but Sparrow backed the idea, also put forward by Ford and Tennari.
"Creating a no-bomb zone would stop the most important tools that have been used to slaughter and terrorize Syrian civilians, especially the children who are the most vulnerable to these toxic gases and whose small bodies are literally ripped apart by the hideous shrapnel inside these bombs," Sparrow said.
The US "policy has to change" and implementing a no-fly zone would lead to "denying Assad ownership of the skies," agreed Representative Ed Royce, committee chairman.
"Syrians would no longer be forced to choose between staying above ground where they could be killed by the shrapnel Assad packs inside his barrel bombs or going below ground where they are more vulnerable to suffocating from chlorine gas," he said.