The Ukrainian Defense Ministry earlier said that all personnel at the depot in the Kharkov region had been evacuated, and that there were no known casualties.
"A depot guard was wounded in the leg by shrapnel from the explosions," Igor Terekhov, vice governor of the region, told a news conference.
According to official reports, the fire at the depot, which held some 95,000 metric tons of ammunition, was caused by forest fires. Local authorities said the explosions at the 500-hectare site made it extremely difficult to extinguish the fire, which could continue for up to three days.
The fire has been declared a state-level emergency. Prime Minister Yulia Tymoshenko said late on Wednesday that 1,400 people had been evacuated and a total of 6,000 would be evacuated from the disaster area.
Soviet-era ammunition depots left on the territory of ex-Soviet states are usually poorly maintained and represent a major threat to the local population.
A fire last month at another Soviet-era depot, in the central Uzbek town of Kagan, resulted in a series of explosions, which killed three people and wounded 21.
In 2004, a fire and explosions at a munitions depot in southern Ukraine lasted for several days and killed five people.
