Residents of the city were told to stay indoors and a state of emergency was issued in parts of the city.
The train had twenty carriages filled with ammonia.
- What we know so far is that one or two wagons derailed and ammonia leaked out, says the city's mayor Vladan Vasic.
The leak caused thick smoke which in turn led to several car accidents on the main road leading to North Macedonia and Greece.
There was an unusual amount of traffic due to the Christmas holiday.
It is not clear what caused the derailment that led to the leak.