Prosecutors described the blast as a possible acts of "sabotage". It was the second major incident affecting a large Ukrainian weapons storage site this year.
Explosions at the depot in the town of Kalynivka, some 175 kilometres southwest of Kiev and about 120 kilometres from the Moldovan border, could be heard every five to 10 minutes and the streets were all but deserted by its 20,000 inhabitants on Wednesday afternoon.
"People suffered heavy damage," a local resident said.
"Some homes had their windows and doors completely blown out."
Initial reports mentioned no fatalities and only two minor injuries from the raging explosions of heavy munitions and air defence missiles at the site.
The Vinnytsia regional administration more than doubled the military's initial estimate of the amount of munitions stored in the depot to 188,000 tonnes after a deeper investigation.
Military general staff spokesman Vladyslav Seleznyov separately told AFP that the arms storage warehouse was "one of the country's largest".
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 scene that hinted strongly at possible Russian or insurgent involvement.
Kiev had blamed a deadly March munitions blast on Moscow and its Russian-backed insurgents fighting Ukrainian forces in the war-wracked east. The Kremlin and its allied militias brushed off the charge as propaganda.
But the ex-Soviet republic's military prosecutor's office said it had launched an investigation into possible "sabotage at a military facility".
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