BELGRADE, Serbia (AP) — A freight train carrying ammonia derailed in eastern Serbia on Sunday, sickening dozens of people and closing a main international highway, officials said.

A state of emergency was declared in the town of Pirot, with authorities telling residents not to leave their homes. Dozens of people were hospitalized, Serbian state media reported.

Because of limited visibility reportedly caused by the leak, several cars crashed on the main highway that leads to Bulgaria at a time of busy traffic because of the Christmas holidays.

Police closed the highway and redirected traffic to local roads. It wasn't immediately clear what caused the train to derail.

Ammonia gas leaks can be flammable and can cause serious injury or even death.