Syria accused insurgents of wounding more than 100 people in a suspected toxic gas attack in Aleppo, which a health official described as the first such assault in the city.
The shells spread a strong stench and caused dozens of people breathing problems on Saturday night in Aleppo, which is under state rule, a monitoring group said.
State news agency SANA said on Sunday that 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.
Syria's foreign ministry urged the U.N. Security Council to condemn and punish "these terrorist crimes".
Rebel officials denied using chemical weapons and accused the Damascus government of trying to frame them.
Russia's defence ministry accused insurgents on Sunday of firing shells filled with chlorine gas at Aleppo from the rebel stronghold of Idlib.
Moscow, Syrian President Bashar al-Assad's key ally, said it would talk to Turkey, which backs some rebel factions and helped broker a ceasefire in the Idlib region.
"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.
Batal called it the first such gas attack hitting civilians in Aleppo city during the conflict, which has raged for more than seven years.
"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.
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 Syrian Observatory for Human Rights, said the shelling in Aleppo wounded 94 people, while government shelling earlier on Saturday had killed nine people in Idlib.
The UK-based monitoring group said two women and seven children were killed in Jarjanaz village in Idlib, where Russia and Turkey have agreed a demilitarised zone.
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 U.N.-OPCW inquiry found the Syrian government used the nerve agent sarin in an April 2017 attack and has also used chlorine several times. It also blamed Islamic State militants for using mustard gas.
Assad's government has repeatedly denied using chemical weapons in the war.