Thirty-nine people in the Russian republic of Chechnya have been admitted to hospital suffering from suspected nerve gas poisoning, officials said.
Chechnya has been torn apart by nearly a decade of war
The first outbreaks of an unknown illness were reported on Friday in the republic's Shelkovskoi district.
Those affected include school pupils, teachers and workers from a school in Starogladovskaya village. Four people are said to be in a serious condition.
Symptoms include breathing problems, fever, headaches and numbness in limbs.
The head of the Shelkovskoi district administration, Khusein Nukhayev, has said the illnesses were likely to have been caused by a poison gas, but was unable to say which type.
A mobile laboratory from the Russian city of Rostov-on-the-Don is carrying out tests.
Chechnya's prosecutor-general Valery Kuznetsov said "to speak about a terrorist act is premature" but "all possibilities are being considered", Russian news agency Tass reported.
Separatist fighters have waged a bitter war against Russian federal forces for a decade, with attacks taking place both inside the republic and elsewhere in southern Russia.