CAIRO -- Several Syrians have died after inhaling poisonous gas released by government forces in rebel-held districts of Homs, local eyewitnesses and activists claimed Monday.
Civilians were admitted to hospital with serious breathing problems after Sunday’s attack, according to doctors and groups who posted what they said was video of the aftermath to YouTube.
The gas is thought to have been a concentrated irritant, but not one of the deadly chemical weapons stockpiled by the regime of Syria president Bashar Assad.
Claims by either side in Syria’s bitter civil war are almost impossible to independently verify because journalists are rarely allowed access to the country.
Mousab Azzawi, chairman of the London-based Syrian Network for Human Rights and a doctor, told NBC News that his organization had received reports from three eyewitnesses on Sunday.
He said field doctors in Homs were seeing patients “losing consciousness, experiencing severe shortness of breath and vomiting.”
“To our understanding, this is similar to poisoning with pesticide,” he said, although he was not aware of any pesticide that could take the form of a gas.
Azzawi added that they were “very concerned and deeply worried” that the attack might be a sign that Assad’s regime might use chemical weapons “on a very small scale.”
Walid Fares, spokesman for the Homs Revolutionary Council -- part of the National Coalition for Syrian Revolutionary and Opposition Forces, the umbrella organization recognized by more than 100 countries including the United States -- issued a statement to NBC News on Monday.
It said “poisonous gases” came from shells fired by government tanks in the districts of Al Bayada and Al Khalideya.
“The shells did not explode but rather emitted a cloud of white smoke and it landed in residential areas… where revolutionaries had gathered and which led to tens being injured,” the statement said.
It said symptoms included “complete absence of vision” as well as nausea, lost consciousness and severe breathing difficulty.
“The initial analysis of the doctors in the hospital confirmed that it is a poisonous gas that contains banned substances,” the statement added, citing videos that claimed to show patients being treated.
'This isn't the first time'
It said there were seven deaths as of early Monday - naming six of the victims - and close to 50 injured.
A third group, the Local Coordination Committees - a network of local opposition councils across Syria - told NBC News: "The LCC has not yet confirmed what the substance was, but doctors in Homs are confirming the use of toxic gases. This isn't the first time; residents of Homs and Zabadani were reporting the use (confirmed) of white phosphorus months ago.”
Two YouTube videos showed patients being treated in hospital for the symptoms of a gas attack. In one, a doctor says in Arabic that the gas is “definitely not Sarin” but is “definitely” poisonous.
Earlier this month, President Barack Obama warned Assad that the use of chemical weapons by his regime would be "totally unacceptable." "If you make the tragic mistake of using these weapons there will be consequences and you will be held accountable," he said.
The alleged gas attack came hours after a senior Israeli defense official said he believed Syria's chemical weapons were still secure despite the civil war.
Amos Gilad told Army Radio that the both sides had become deadlocked but there was no sign of Assad heeding international calls to step down, according to a Reuters report.
"Suppose he does leave, there could be chaos ... in the Middle East you never know who will come instead. We need to stay level-headed; the entire world is dealing with this. At the moment, chemical weapons are under control," Gilad said.
Reuters contributed to this report.
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