Fighting raged Sunday on the western edges of the Syrian city of Aleppo, with rebels trying to split a Russian-backed government force laying siege to the eastern half of the city by attacking it from the rear.
Monitors say at least 41 civilians have been killed in the latest offensive, and Syria's state news agency SANA said 35 others were suffering from the effects of "toxic gases" in two government-controlled districts.
The SANA report linked the gas attack to what it called "terrorist organizations" affiliated with Turkey and Saudi Arabia. For their part, rebels accused government forces of shelling a separate rebel-held district with chlorine shells.
The head of Aleppo University Hospital, speaking on state television, identified the fumes as toxic chlorine gas. No deaths were linked to the gas, which monitors from the Syrian Observatory for Human Rights described as creating symptoms of suffocation.
Rebels and monitors said much of Sunday's fighting centered on a battle for a huge apartment housing project in a western district, which, if captured, would bring rebels within several kilometers of the center of government-controlled western Aleppo.
Government reportedly moving in reinforcements
Rebels have used suicide car bombers and heavy artillery in their attack from the west, which was launched on Friday. Syrian Observatory chief Rami Abdurrahman said the government moved to counter the rebel offensive by calling in 1,000 artillery troop reinforcements from central Syria in apparent preparation for a counterattack.
Aleppo, the country's largest city, has been divided among government troops, rebel militias, Islamic extremists and Kurdish fighters since 2012 - a year after the country erupted into civil war. Much of the city now lies in ruins.
The government of embattled President Bashar al-Assad has repeatedly been accused of attacking rebels with crude chemical bombs.
Earlier this year, the Syrian-American Medical Society (SAMS) said chemical weapons attacks have killed nearly 1,500 people since civil war broke out in March of 2011.
The SAMS report, released in March, documented 161 chemical attacks in Syria, and linked the vast majority of them and the resulting civilian casualties to the Assad government.