Syrian Information Minister Omran al-Zoabi said at least 25 died and 86 others were injured in the attack, which he blamed on the rebels, Russia Today reported.
Syrian rebel commander Qassim Saadeddine denied the government's accusations and claimed the Syrian military had fired Scud missiles containing chemical agents on Khan al-Assal.
The opposition group Syrian Observatory for Human Rights reported 26 people died in the attack, including 16 who died at the scene and 10 at hospitals.
RT reported the watchdog group Organization for the Prohibition of Chemical Weapons had not independently confirmed the use of chemical weapons in Syria.
The Washington Post reported the U.S. government said it had no immediate evidence a chemical weapons attack had occurred but was looking into the allegations.
White House press secretary Jay Carney told reporters at his daily briefing the administration was "deeply skeptical" the rebels were responsible if chemical weapons were deployed, the newspaper said.
But Carney called the reports of chemical weapons use a "serious concern" that "is and would be totally unacceptable" if true.
The Syrian Arab News Agency said rebels, which the state-run agency calls terrorists, fired a rocket containing chemical substances in Khan al-Assal area outside Aleppo.
Al-Zoabi said Turkey and Qatar bore "legal, moral and political responsibility" for the attack because of their support of rebel groups fighting to oust President Bashar Assad, Russia Today said.
Medical sources in Khan al-Assal said people suffering from suffocation and toxicity due to the exposure of unknown gases were being treated, China's Xinhua News Agency reported.
In other areas, including Idlib, Homs and Hamadiya, intense fighting between Syrian regime forces and rebel troops was reported. SANA said the army destroyed a number of terrorist hideouts and vehicles mounted with machine-guns.
Earlier in Istanbul, Turkey, Syria's main opposition group picked a naturalized U.S. citizen, Ghassan Hitto, 49, a Syrian-born telecommunications executive, as its first prime minister.
Some coalition members said the election would send a message to Assad the opposition could challenge his rule politically as well as militarily, The Wall Street Journal reported.
It was unclear how many rebel brigades would recognize the interim government as a legitimate authority, The New York Times said.
Hitto's election came hours after Syrian warplanes attacked suspected rebel hideouts across the border in eastern Lebanon, Lebanese and U.S. officials said.
The jet fighter and helicopter assault marked the first anti-rebel cross-border attack by the Assad regime since the Syrian conflict began two years ago, the Times said.
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