Ukraine said Thursday that a fire raging at a munitions depot in the country's east, which the military blamed on an "act of sabotage", could continue for up to a week.
As of Thursday afternoon 20,000 people living within a 10-kilometre (six-mile) radius of the depot had been evacuated.
"We can evaluate everything as the intensity of the fire decreases," Ukrainian Prime Minister Volodymyr Groysman said in televised comments. "This can take up to one week."
Groysman said 550 emergency workers, medics and police officers had been dispatched to the scene to keep the fire from spreading to nearby populated areas.
However the fire's reach was still expanding, with the emergency workers unable to contain it.
Earlier in the day the crackling sound of detonations thought to be exploding munitions could be heard from some 20 kilometres away.
The fire broke out overnight in the town of Balakliya at a depot where missiles and munitions were kept, causing them to detonate.
Ukraine's Chief Military Prosecutor Anatoliy Matios said in a statement that the blaze had been caused "as a result of an act of sabotage."
Matios said witnesses had heard a sound resembling a drone in flight before the blasts began in Balakliya, which is located some 100 kilometres from the pro-Russian insurgent stronghold of Donetsk.
- Cloud of smoke -
Ukraine Defence Minister Stepan Poltorak said authorities were considering the theory that "explosive devices dropped from an unmanned aerial vehicle" could have caused the fire, Interfax Ukraine news agency reported.
He added that there appeared to be no casualties in the explosions.
A 54-year-old woman sustained shrapnel wounds and was transported to a local hospital, the regional police force said in a statement.
An AFP reporter who accessed Balakliya saw it almost completely deserted following evacuation, with the silence broken by occasional blasts while clouds of grey smoke rose above residential buildings.
In Yakovenkove, a village five kilometres north of town, locals said their homes had been hit by shapnel from the burning depot.
Ukraine President Petro Poroshenko has ordered increased security controls at the nation's military facilities, presidential spokesman Svyatoslav Tsegolko said.
The Ukrainian military has been fighting pro-Russian separatists in the country's east since April 2014 in a conflict that has killed more than 10,000 people.
The depot in Balakliya housed munitions and arms used by Ukrainian troops fighting the insurgents, according to Ukrainian television.
At least two civilians were killed and eight people injured in a similar incident in October 2015 in the government-held city of Svatove, some 60 kilometres outside rebel-held territory.
Soviet-era military warehouses usually contain large amounts of old weapons and ammunition that sometimes are stored in poor conditions.