Dozens of people have been killed in
air strikes and a suspected gas attack near the Syrian city of Palmyra,
monitoring groups say.
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.