"Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress," said Dr. Bart Janssens, MSF director of operations.
Patients were treated using atropine.
"MSF can neither scientifically confirm the cause of these symptoms nor establish who is responsible for the attack," said Janssens. "However, the reported symptoms of the patients, in addition to the epidemiological pattern of the events --- characterized by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers --strongly indicate mass exposure to a neurotoxic agent."
How will the world respond?
On Saturday, world leaders weighed their options.
A Downing Street spokesperson said UK Prime Minister David Cameron -- who also talked with his Canadian counterpart Stephen Harper -- spoke Saturday to Obama.
"They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options," the spokesperson said.
The White House issued a statement on the same conversation, saying the two leaders agreed to "consult closely regarding this incident, as well as possible responses by the international community to the use of chemical weapons."
If the claims that Syria used chemical weapons prove true, a speedy response will be needed to prevent another such attack, U.S. Defense Secretary Chuck Hagel said.
Hagel's comments Friday came after a senior Defense Department official told CNN that military planners have updated Syrian target lists.
And it was disclosed that a fourth U.S. ship armed with cruise missiles has arrived in the eastern Mediterranean Sea.
Hagel addressed the issue aboard a military plane headed to Malaysia.
"We will determine at some point here very shortly what did happen," he said, according to a transcript posted on the Defense Department's website.
"If, in fact, this was a deliberate use and attack by the Syrian government on its own people using chemical weapons, there may be another attack coming," Hagel said. "A very quick assessment of what happened and whatever appropriate response should be made."
Hagel said the American military provided Obama "with options for all contingencies, and that requires positioning our forces, positioning our assets to be able to carry out different options, whatever options the president might choose."
He did not specify what those options might include.
Hagel predicted other nations would lend their support if the investigation finds that the Syrian government used chemical weapons against its people.
"This is an international community issue; it violates every standard of international behavior," he said. "That said, the United States has never given up its own sovereign right to protect its own interests."
The president has said he does not anticipate using ground forces in Syria. Other military options could include airstrikes by fighter jets or cruise missiles.
The Navy destroyer USS Ramage has arrived in the region, a defense official said late Friday. It was intended to replace the USS Mahan, but the Mahan will remain temporarily along with the USS Gravelly and USS Barry. All four are equipped with cruise missiles.