Syrian state media and a Syrian watchdog say about 100 people in Aleppo have been hospitalized, complaining of breathing difficulties.
The Britain-based Syrian Observatory for Human Rights says 94 people were treated and 31 remain hospitalized.
Russia, the top backer of Bashar al-Assad's government in Syria, said Sunday it believes rebel forces fired missiles with chlorine. Russia's Defense Ministry said it launched air strikes on the Syrian rebels in retaliation.
"Air strikes were carried out by Russian air force planes," Defense Ministry spokesman Igor Konashenkov said in comments reported by TASS state news agency, adding that "as a result of the strikes, all of the rebel fighter targets were destroyed."
SANA, Syria's state news agency, said Sunday that 107 people were injured in Aleppo after militants in Idlib hit areas with projectiles that probably contained chlorine. Russia has also blamed the injuries on rebel-launched missiles with chlorine.
Rebel officials, however, denied the charges and accused Syria of trying to frame them. "We at the National Liberation Front deny the criminal, lying regime's allegations that revolutionaries targeted the city of Aleppo with any missiles and especially not any containing chlorine gas," the rebels said Sunday.
The Syrian Foreign Ministry urged the U.N. Security Council Sunday to take action against whoever conducted the strikes. The ministry said the Syrian government "calls on the Security Council to immediately and strongly condemn these terrorist crimes... [and take] deterrent, punitive measures against the actions and regimes that support and fund terrorism."
Zaher Batal, the head of the Aleppo Doctors Syndicate told Reuters, "We can not know the kinds of gases, but we suspected chlorine and treated patients on this basis because of the symptoms."
In addition to difficulty breathing, symptoms included eye inflammation, shivering and fainting.
The West has accused Damascus of using chemical weapons on several occasions, something Syria has consistently denied.