More than 100 people were wounded in a suspected toxic gas attack in Syria's Aleppo late on Saturday, which a health official said was the first such assault in the city.
The Syrian government and its ally Russia blamed the attack on insurgents, which rebel officials denied.
Russia's defence ministry said on Sunday its warplanes bombed militants who had fired chlorine gas at Aleppo.
Moscow said it would talk to Turkey, which backs some rebel factions and helped broker a ceasefire in the insurgent stronghold of Idlib.
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A monitoring group said air strikes hit rebel territory in the northwest on Sunday for the first time since Russia and Turkey agreed a buffer zone there in September.
In Aleppo city which the government controls, the shells had spread a strong stench and caused breathing problems, the Syrian Observatory for Human Rights also said.
State news agency SANA said on Sunday 107 people were injured, including children, after militants hit three districts with projectiles containing gases that caused choking.
It marks the highest such casualty toll in Aleppo since government forces and their allies clawed back the city from rebels nearly two years ago.
"We can not know the kinds of gases but we suspected chlorine and treated patients on this basis because of the symptoms," Zaher Batal, the head of the Aleppo Doctors Syndicate, told Reuters.
Patients suffered difficulty breathing, eye inflammation, shivering and fainting, he said. Hospitals had discharged many people overnight.
Syria's forensic medicine general director, Zaher Hajo, told The Associated Press that all but 15 of the 105 people who were treated have been discharged. He said two people who were in critical condition have improved.
The Observatory said 94 people were treated, with 31 remaining in hospitals.
The Britain-based Observatory, which relies on a network of activists and contacts throughout Syria, said the airstrikes hit the Rashideen district on the western outskirts of Aleppo and the village of Khan Touman south of the city.
Batal said this was the first gas attack against civilians in the city since the conflict erupted more than seven years ago.
Syrian state media meanwhile reported that rebels shelled the Christian village of Mahradeh in northwestern Syria, causing material damage but no casualties.
STRETCHERS AND OXYGEN MASKS
No group has claimed the attack so far. Russia said it came from territory that former Al-Qaida militants control in Idlib.
"The explosive (shells) contain toxic gases that led to choking among civilians," Aleppo police chief Issam al-Shilli told state media.
Pictures and footage on SANA showed medical workers carrying patients on stretchers and helping them with oxygen masks.
State media said the army hit back at militants near the city but did not elaborate. Syria's foreign ministry urged the UN Security Council to condemn and punish the attack.
Abdel-Salam Abdel-Razak, an official from the Nour el-Din al-Zinki insurgent faction, said rebels did not own chemical weapons or have the capacity to produce them.
"The criminal regime, under Russian instructions, is trying to accuse the rebels of using toxic substances in Aleppo. This is purely a lie," he tweeted.
Abu Omar, a Failaq al-Sham spokesman, accused Damascus of trying to create "a malicious charade" as a pretext to attack rebel towns.
The UK-based Observatory said government shelling earlier on Saturday had killed two women and seven children in a village in Idlib.
The Russian-Turkish deal in September had staved off an expected army offensive against the Idlib region, including nearby parts of Aleppo and Hama provinces.
The truce brokered by Russia and Turkey, which supports the rebels, has been repeatedly violated, but until Sunday there had been no airstrikes.
The dominant force among an array of factions holding sway in Idlib is Tahrir al-Sham, an Islamist alliance led by fighters formerly linked to al-Qaeda.
A past UN-OPCW inquiry found the Syrian government used the nerve agent sarin in April 2017 and has also used chlorine several times. It also blamed Islamic State for using mustard gas.
Assad's government has repeatedly denied using chemical weapons in the war.