The main Western-backed opposition group, the Syrian National Coalition, said the poison gas attack Friday hurt dozens of people in the village of Kfar Zeita in the central province of Hama. It did not say what type of gas was used.
The Britain-based Syrian Observatory for Human Rights reported that people suffered from suffocation and breathing problems after the attack, apparently conducted during air raids that left heavy smoke over the area.
"Regime planes bombed Kafr Zita with explosive barrels that produced thick smoke and odors and led to cases of suffocation and poisoning," Observatory director Rami Abdel Rahman said.
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But state-run Syrian television blamed members of the al-Qaeda-linked Nusra Front for using chlorine gas at Kfar Zeita.
"There is information that the terrorist Nusra Front released toxic chlorine… leading to the death of two people and causing more than 100 people to suffer from suffocation," it said.
The TV report claimed the Nusra Front is preparing for another chemical attack against the Wadi Deif area in the northern province of Idlib, as well as another area in Hama. It did not explain how it knew the Nusra Front's plans.
An amateur video posted online by opposition activists showed a hospital room in Kfar Zeita that was packed with men and children, some of whom breathing through oxygen masks. On one bed, the video showed six children on a bed, some appearing to have difficulty breathing while others cried.
The video appeared genuine and corresponded to other Associated Press reporting of the attack.
Chemical weapons have been used before in Syria's 3-year-old conflict. In August, a chemical attack near the capital, Damascus, killed hundreds of people.
The US and its allies blamed the Syrian government for that attack, which nearly sparked Western airstrikes against President Bashar Assad's forces. Damascus denied the charges and blamed rebels of staging the incident.
AFP contributed to this report.