"A fire broke out ... as a result of an act of sabotage at a depot in the town of Balakliya where missiles and munitions were kept," Ukraine's Chief Military Prosecutor Anatoliy Matios said in a statement. "The fire led to the detonation of munitions."
Authorities said Thursday morning that there appeared to be no casualties in the explosions and that some 20,000 people living in a five-kilometre radius were being evacuated from the area.
Ukraine's security services said they had opened an investigation into a suspected act of "diversion".
The Ukrainian military has been fighting pro-Russian separatists in the country's east since April 2014 in a conflict that has killed more than 10,000 people.
The depot in Balakliya, located some 150 kilometres from eastern Ukraine's rebel-held regions, housed munitions and arms used by Ukrainian troops fighting the insurgents, according to Ukrainian television.
At least two civilians were killed and eight people injured in a similar incident in October 2015 in the government-held city of Svatove, some 60 kilometres outside rebel-held territory.
Soviet-era military warehouses usually contain large amounts of weapons and ammunition that sometimes are stored in poor conditions.
