Some 20,000 people are being evacuated after a series of explosions at a massive arms depot in eastern Ukraine described by officials as sabotage.
The base in Balakliya, near Kharkiv, is around 100km (60 miles) from fighting against Russian-backed separatists.
The dump is used to store thousands of tonnes of ammunition including missiles and artillery weapons.
Rescue teams are overseeing a huge evacuation effort for people living in the city and nearby villages.
The total area of the dump spans more than 350 hectares, the military says.
Everyone within a 10km (6 miles) radius of the dump is being evacuated, the Interfax news agency quoted an aide to President Petro Poroshenko as saying.
Munitions from the depot are used to supply military units in the conflict zone in nearby Luhansk and Donetsk, reports say.
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EPA
A huge cloud of smoke could be seen billowing above the ammunition depot of the Ukrainian armed forces on Thursday
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DSNS
By daylight smoke billowed across the sky in Balakliya
The authorities are investigating various ways the explosions may have been caused, Defence Minister Stepan Poltorak said, including the possibility of an explosive device being dropped from a drone.
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A drone was reported to have been used an earlier attempt to set the facility on fire in December 2015.
Mr Poltorak said that there were no reports that civilians or serviceman had been killed or injured in the latest incident and that airspace had been closed within a 50km (31 miles) radius of Balakliya.
More than 9,700 people have died in the conflict which erupted in 2014 when Russia annexed Ukraine’s southern Crimea peninsula. Pro-Russian rebels later launched an insurgency in the east.
Ukraine munitions blasts prompt mass evacuations