GROZNY -- Authorities closed schools in part of Chechnya on Wednesday after a mysterious poisoning sickened at least 70 people, most of them schoolchildren, officials said.

Sergei Kozhemyaka, a regional official in the Emergency Situations Ministry, said the schools would remain closed until further notice.

Medical workers from Moscow have been taking blood samples trying to establish the cause of the illness. Among the leading theories is poisoning from a type of nerve gas.

Dozens of school-age children from four towns in Chechnya's northeastern Shchyolkovsky region have been hospitalized, most within the past few days, Gazeta.ru reported.

The afflicted, who have also included teachers and school workers, have reported breathing trouble and headaches.

Deputy Chechen Health Minister Zaur Musluyev said 53 of those sickened were children.

Umar Akhyadov, chief doctor of the Chechen Health Ministry's emergency medicine center, was quoted by Interfax as saying that the number could rise.

Rebels who have been fighting federal forces in Chechnya for most of the past decade have committed a series of terrorist attacks in Chechnya and other parts of Russia, including the deadly seizure of hostages in a school in the town of Beslan in 2004. The attack killed 331 people, more than half of them children.

Chechen Prosecutor General Valery Kuznetsov said Tuesday that "to speak about a terrorist act is premature" but "all possibilities are being considered," Itar-Tass reported.