Members of Congress on Wednesday heard testimony from a Syrian doctor who said that Bashar al-Assad's government continues to use chlorine as a chemical weapon against civilian populations in rebel-controlled areas of the country.

Dr. Mohamed Tennari, who runs a field hospital in Sarmin, in Syria's Idlib province, presented evidence of the attacks, compiled by the Syrian American Medical Society (SAMS), to the House Foreign Affairs Committee. SAMS says it has counted 31 chlorine attacks in the province between March 13 and June 9.

Although Chlorine is not considered a chemical weapon in its own right — unlike the Sarin stockpiles which are believed to have been removed from Syria — it is deemed illegal under international law once weaponized, something Syrian civilians, rescue workers and doctors have claimed is increasingly the case in Idlib province, where rebels have made significant gains this year.

"Over the last four years, we have seen horrific and indiscriminate violence against civilians, in the form of barrel bombs, missile strikes, shelling and more," Tennari told the committee. "However, three months ago, my province of Idlib began experiencing a new type of attack: chlorine-filled barrel bombs."

Tennari offered his testimony along with Farouq Habib, Syria program manager for Mayday Rescue, a partially western-funded group that trains first responders; Annie Sparrow, a professor of global health at Mount Sinai's Icahn School of Medicine; and Robert Ford, who served as the American ambassador to Syria from 2011 until 2014.

Ford, like the other members of the panel, pushed the representatives to consider measures to ensure the protection of civilians, which could include a no-fly zone. Such a step has long been floated in diplomatic circles, but has seen little support up until now from the American government, whose military would likely take a lead in such an effort. What a no-fly zone would specifically entail, and who among the myriad rebel groups it might benefit, has also not been fully established.

"We don't know how long a no-fly zone over Syria would last," cautioned Ford. He said in lieu of one, it should be possible "for us and our regional allies to help Syrian rebels hit with rockets or mortars from a distance the Syrian government airfields where aircraft are based."

The ambassador estimated Syrian government forces were responsible for between 150,000 and 200,000 deaths during the country's four-year civil war. The Islamic State, he added, was tied to at most 5,000 killings. Ford did not say how he arrived at those numbers. The UN estimates more than 200,000 people have died during the conflict.

Last fall, the Organization for the Prohibition of Chemical Weapons (OPCW) said it had found "compelling confirmation" that chlorine had been used "systematically and repeatedly" in Syria. Western governments and locals say the only culprit could be the Syrian military because it alone possesses a fleet of helicopters, from which the jerry-rigged chlorine canisters are reportedly dropped. The OPCW, however, is not mandated to assign blame, and the Syrian government has always denied it is dropping chlorine. Assad has also repeatedly rejected the allegation that his forces use rudimentary and indiscriminate barrel bombs that have caused widespread carnage in rebel-controlled areas.

Among the dossier given to representatives were pictures and videos of attacks, including from one that took place on March 16.

"As soon as I left my house, I could smell a bleach-like odor," said Tennari. "When I arrived at the hospital, a wave of people had already begun to arrive. They were all experiencing symptoms of exposure to a choking agent like chlorine gas."

The family of Waref Taleb, including his wife, mother and three children, were brought to the hospital after a chlorine canister allegedly landed in their basement apartment. They would later die.

"One of the barrel bombs fell through a shaft in their home filling the ventilation with chlorine as it broke open," said Tennari. "Their basement became a makeshift gas chamber."

"I'm not usually speechless, but after watching those picture of the children dying, I'm speechless," said Rep. Eliot Engel (D-NY).

The same video of the aftermath of the March 16 attack was shown to the UN Security Council in April, reportedly bringing some members to tears. The Council however, has proven unable to take decisive action to stop any attacks.

In March, the 15-member body passed a resolution condemning chemical weapons use in Syria, but only threatened further steps if further attacks are proven. Russia, Syria's closest ally on the Council, has similarly denied there is any evidence that warrants the assigning culpability for the attacks. Meanwhile, US-led efforts to establish a separate mechanism for attribution, has been mired in negotiations. Council diplomats say Russia has objected to many elements in the US-proposal.

"Let's be clear: only Assad's forces have helicopters," said committee Chairman Rep. Ed Royce (R-CA). "This can't just keep going on and on. If nothing is done, the human tragedy in Syria and the region will reach depths the world hasn't seen in generations."

On Tuesday, Secretary of State John Kerry told reporters that "everyone's patience is wearing thin," after the repeated attacks, and said he was "confident" that Russian Foreign Minister Sergei Lavrov would speak to Assad about them.

At the hearing, other representatives, including Jeff Duncan (R-SC), questioned the legality of any military actions the US and its allies might take to stop the bombings. Unlike NATO's 2011 intervention in Libya, the UN Security Council has not offered a Chapter-7 mandate for military involvement in Syria. Diplomats quietly concede that the subsequent chaos in Libya has given Russia ammunition to oppose any kind of involvement.

Congressman Ted Poe (R-TX) asked Ford to tender another option. "Why doesn't the United States, if Assad is so bad, if he's killed 200,000 of his people… why don't we just assassinate him?"

Ford said assassinations were not part of US policy.

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