Three Syrian hospitals have received around 3,600 patients displaying "neurotoxic symptoms," 355 of whom have died, Doctors Without Borders (Médecins Sans Frontières, or MSF) said Saturday.
The humanitarian group's statement follows reports that as many as 1,300 people were allegedly killed in massive chemical attacks near Damascus Wednesday that opposition groups say were carried out by President Bashar Al-Assad's forces.
US President Barack Obama was reviewing Saturday a possible response to an alleged chemical "massacre" in Syria, as a top UN officials press Damascus for an investigation into the alleged attacks.
The Syrian government has strongly denied the allegations, but has yet to accede to demands that UN inspectors already in the country be allowed to visit the sites of the alleged attacks.
As US Defense Secretary Chuck Hagel suggested the US Navy was moving forces closer to Syria, Obama met his top national security advisors to discuss the US response to the alleged use of chemical weapons, a White House official said.
"We have a range of options available, and we are going to act very deliberately so that we're making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria," the official said.
On Friday, Hagel said his department "has a responsibility to provide the president with options for all contingencies," which includes "positioning our forces ... to be able to carry out different options — whatever the president might choose."
But he declined to provide any details on the deployment of those forces, as the Obama administration reportedly contemplated cruise missile strikes against Assad's forces.
The New York Times cited a senior administration official as saying Washington was looking at NATO's air war over Kosovo in 1999 as a blueprint for strikes on Syria without a UN mandate.
Obama has repeatedly warned that the use of chemical weapons by Syrian forces was a "red line" that could bring about a more strident Western response to the 29-month-old civil war.
But he has also said Washington must be wary of costly and difficult foreign interventions.
On Thursday, French Foreign Minister Laurent Fabius said that if the massacre was confirmed, "there must be a reaction, a reaction that could take the form of a reaction with force."
On Saturday, during a visit to the West Bank, Fabius said that a "chemical massacre" had occurred and that "the Bashar regime is responsible."
Fabius's remarks followed similar accusations from British Foreign Secretary William Hague, who said Syria carried out a "large scale" chemical attack and insisted that Damascus give UN inspectors access to the suspected sites.
But Damascus ally Iran blamed the rebels and warned the West against military intervention.
"There is proof terrorist groups carried out this action," Iranian foreign ministry spokesman Abbas Araqchi said, without giving details.
Warning against Western military intervention in the conflict, Araqchi said "there is no international authorisation for" such an action.
"We warn against any actions or statements that could create more tension in the region. I hope that White House officials show enough wisdom not to enter into such dangerous tumult."
Meanwhile, UN Undersecretary General Angela Kane was in Damascus, tasked by Secretary General Ban Ki-moon with establishing the terms of an inquiry.
Ban is determined to "conduct a thorough, impartial and prompt investigation" into the chemical attack claims, his spokesman said.
On Friday, Syria's main opposition group, the National Coalition, pledged to guarantee the safety of UN inspectors, but warned that the "clock is ticking" before alleged evidence vanishes.
Syria has yet to say if it will let the UN experts — on the ground since 18 August — conduct a probe.