Police said the flames caused artillery shells at the facility to explode one after the other. PHOTO: AFP
It was the second major incident affecting a Ukrainian weapons storage site this year.
Kiev blamed the first one in March on Moscow and its Russian-backed insurgents fighting Ukrainian forces in the war-wrecked east -- a charge both sides denied.
The pro-Western former Soviet nation's military prosecutor's office said Wednesday that it was opening an investigation into possible "sabotage at a military facility".
Immediate reports mentioned no fatalities and only two minor injuries from the blasts.
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An AFP reporter at the scene said blasts in the town of Kalynivka in the Vinnytsya region located some 175 kilometres (110 miles) southwest of the capital Kiev could be heard every five seconds on Wednesday morning and that the streets of the town of 20,000 were nearly deserted.
"People suffered heavy damage," a local resident who agreed to identify herself only as Antonina told AFP.
"Some homes had their windows and doors completely blown out," she said.
President Petro Poroshenko underscored the seriousness of the situation by telling his top military brass and Prime Minister Volodymyr Groysman to report to him directly after visiting the site.
"This is the arsenal of the Ukrainian army, and I think it was no accident that it was destroyed," Groysman said in televised remarks from the site.
The army's high command wrote on Facebook that the fire broke out at around 10pm (1900 GMT) Tuesday.
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Police spokesperson Yaroslav Trakalo told the Pravda news outlet the flames caused artillery shells at the facility to explode one after the other.
The national police said more than 30,000 people had been evacuated from areas immediately surrounding the military site.
"In addition, 180 patients were evacuated from Vinnytsia area hospitals," Ukraine's emergency ministry said.
The extent of the damage was not clear but local media described the depot as one of the country's largest.
Officials also closed off the surrounding airspace to air traffic as a precaution.
"Airspace has been closed in a radius of 50 kilometres (30 miles) around the area of the fire," Deputy Infrastructure Minister Yuriy Lavrenyuk wrote on Facebook.
The last major arms depot fire killed one person in the eastern town of Balakliya in March.
Authorities at the time pointed the finger at Moscow and Russian-backed insurgents fighting Ukrainian troops in the eastern rustbelt in a war that broke out in April 2014 and has claimed more than 10,000 lives.
Some officials in Kiev then mooted the possibility that the fire was caused by explosives dropped from a drone.
Both the Russian-backed forces and the insurgents dismissed the charge outright.
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