The targeted area is controlled by the so-called Islamic State group and has been under heavy bombardment from government-aligned forces.
Dozens of people are said to have been killed during the combined assaults.
IS members recaptured the city on Sunday, hours after Russian air strikes appeared to have driven them back.
Sources in Hama province reported seeing dead bodies with no visible injuries, according to the UK-based group Syrian Observatory for Human Rights.
The death toll was put at at least 40 by one citizens' group in Hama province.
The Syrian government and its Russian backers have previously denied using chemical weapons.
IS was previously driven out of the ancient desert city of Palmyra in March with the support of Russian air strikes, but the jihadist group seized it again in a sudden assault that started last week.
The surprise setback for Syrian government forces came as they and their allies turned their attention to fighting local opposition forces in Aleppo and Damascus.
IS destroyed a number of monuments and beheaded the archaeological director during its 10-month occupation of the Unesco World Heritage site and the adjacent city of Tadmur.
Two 2,000-year-old temples, an arch and funerary towers were left in ruins.
The jihadist group, which has also demolished several pre-Islamic sites in neighbouring Iraq, believes that such structures are idolatrous.
Dozens of people have been killed in air strikes and a suspected gas attack near the Syrian city of Palmyra, monitoring groups say.