"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.
"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.
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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.
"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.