Blasts at the defense ministry's arsenal at Kaynivka, about 170 miles west of the Ukrainian capital of Kyiv, began late Tuesday. Ukrainian Security Services spokeswoman Olena Hitlyanska said Wednesday that investigators are treating the incident as an act of sabotage.
Ukraine's National Police, about 600 National Guardsmen and hundreds of police officers from surrounding communities led the evacuation from towns in a six-mile radius of the explosion.
About 1,200 firefighters are involved in extinguishing the fire. Buses and trains were used to remove civilians from the area, and rail traffic in the area was later rerouted. The military unit overseeing the arsenal was also closed as personnel left.
A report by the General Staff of the Armed Forces of Ukraine noted that nearly 22,000 civilians and 500 families of servicemen were evacuated during the night.
Nearby roads and rail lines were closed after the evacuation, as was air traffic within a 30-mile radius of the blast. Power and gas lines to local towns were cut off, the Kyiv Post reported Wednesday.