Serbian authorities have urged residents to stay indoors until the air clears.
At least 51 people have been poisoned after a train derailed in southeastern Serbia, releasing ammonia into the air.
Serbia’s Interior Ministry said on Monday the ammonia was released from a wagon of the derailed freight train in the city of Pirot.
Police warned residents to stay in their homes.
The mayor of Pirot, Vladan Vasic, declared a state of emergency and classes were suspended for one day at all schools.
"The main recommendation is that people do not leave their homes, do not turn on air conditioners so that outside air does not enter and we all wait for that cloud of smoke to pass," said Vasic.
"Seven people were taken to (a hospital) in Nis," the nearest big city, he added.
He said there was ammonia in 20 wagons and the leaking started from two wagons.
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