The blasts at the Balakliya military base – which is the largest in the country – led to the evacuation of more than 30,000 people and forced the region’s airspace to be closed.
Two people were injured.
Some 83,000 tons of artillery shells are said to be stored at the depot, which lies 168 miles from the capital, Kiev.
The explosions late on Tuesday triggered a huge fireball which lit up the skies over the site, less than 100 miles from the frontline of hostilities between Ukrainian government forces and Russian and separatist troops.
The war has killed over 10,000 people since 2014. More than 1,000 soldiers joined dozens of firefighters in working through the night to try to tame the huge fire.
A defence ministry spokesman said a drone had most likely been used to start the blaze.
“A soldier from one of the guard posts reported hearing a strange sound,” he said.
“Several seconds later the explosions began.”
During a visit to the scene, Prime Minister Volodymyr Groysman said “external factors” were to blame.
He said footage showed that 70 per cent of the ammunition had survived, thanks to routine safety measures already in place.