The explosions occurred at the ammo depot near Ichnia in Chernihiv Oblast, about 100 miles east of Kiev.
The arsenal, covering a little more than a quarter of a square mile, has 176,000 pounds of ammunition.
Authorities also cut off electricity and gas supplies to the area and closed about 20 miles of airspace around the site, as the State of Emergency Service of Ukraine launches operation to stop the fire.
The explosions have led to evacuations of 12,000 people so far, including patients from hospitals, authorities said.
Ukraine’s Defense Ministry suggested sabotage and said there were two simultaneous explosions in one area of the depot, followed moments later by two more in another area.
“At present, due to explosions, the military prosecutor’s office of the Central Region of Ukraine has started criminal proceedings under Part 3 of Article 425 of the Criminal Code of Ukraine (careless attitude to military service),” a military prosecutor’s office of the Central Region of Ukraine said.
A pre-trial investigation is underway by the Ukranian Security Service.
No casualties or injuries were reported and the material damage is being assessed, Interfax news agency reported.