The rebels denied using chemical weapons and accused the Damascus government of trying to frame them.
This comes after government shelling killed nine people on Saturday in Syria's rebel-held Idlib, while state media said insurgent shells wounded dozens in Aleppo causing breathing problems.
Around 100 Syrians have been hospitalised with breathing difficulties in Aleppo, state media and a monitor said Sunday, after allegations rebels fired "toxic gas" on the regime-held city the previous day.
The conflict has killed more than 360,000 people and displaced millions, according to the Observatory.
There has been no major offensive in northern Syria since September, when Russian Federation and Turkey agreed to create a buffer zone to separate government forces from rebel fighters in Idlib province.
"According to preliminary information, confirmed particularly by the victims' symptoms of poisoning, the shells that were fired at residential areas of Aleppo were filled with chlorine", defence ministry spokesman Igor Konashenkov said at a briefing, quoted by Russian news agencies. The targeted area is opposition-held and home to extremist groups opposed to the truce such as the al-Qaida-linked Horas al-Din, which has described the deal as a "great conspiracy", and the Ansar al-Din Front. There are no deaths, al-Shali said.
Aleppo Governor Hussein Diab visited the injured at the hospital.
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The September 17 deal was meant to protect three million inhabitants in the Idlib region, more than half of which is held by an alliance led by jihadists of Al-Qaeda's former Syria affiliate.
He told state TV that 41 people had been admitted and accused rebels of using poisonous gas in the missiles they lobbed at the Aleppo neighborhood. The Syrian regime and its Russian ally are threatening an offensive to retake the northwestern province of Idlib, Syria's last rebel bastion.
"We cannot know the kinds of gases but we suspected chlorine and treated patients on this basis because of the symptoms", said Zaher Batal, head of the Aleppo Doctors Syndicate.
Rebel spokesman Musafa Sejari said the government is seeking to undermine the ceasefire deal.
Al-Buraykat, the settlement mentioned in the Russian statement, is not included in maps.The civilians and active Free Syrian Army (FSA) groups told Anadolu Agency that they didn't know about any place addressed as "Al-Buraykat".
Both rebels and the government have blamed each other for chemical attacks and other war crimes throughout the seven-year war, with each side accusing the other of creating fake narratives and carrying out false-flag operations.
Past year and in April, the allegations caused the United States to carry out punitive strikes in Syria. Syria's foreign ministry urged the U.N. Security Council to condemn and punish the attack. The UN-OPCW team also accused the Daesh terrorist group of using mustard gas twice, in 2015 and 2016.
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