Security services said they were investigating "possible sabotage" in the incident at a defence ministry depot near the village of Druzhba, in the northern Chernigiv region around 135 kilometres northeast of Kiev.
Emergency services said they had no information of any fatalities, while regional authorities said more than 60 people required medical help for smoke inhalation. Grey and white smoke billowed and a fireball briefly lit up the sky above the depot, an AFP photographer saw, while explosions rumbled every one to two minutes.
A defence ministry official said Tuesday afternoon the fire had been localised and "the intensity of the explosions is continuing to go down." The fire and explosions began around 3:30am local time (0030 GMT) at the Number Six depot, which covers an area of about 700 hectares (1,700 acres), the emergency services said.
Four explosions went off in different parts of the depot before a fire broke out, said the deputy head of Ukraine's General Staff, Rodion Tymoshenko, adding that "this mean it's most likely sabotage," implying Russia's involvement.
Kiev forces have been fighting pro-Russian separatists in the east of the country since 2014 in a conflict that has killed more than 10,000 people. More than 12,000 people were evacuated from the area at risk, the emergency services said.
In the nearby small town of Ichnya, streets appeared deserted and were patrolled by police and the National Guard.
Shells "flew over the house at three o'clock in the morning, whistling overhead," said Ichnya resident Pavlo Grebenyuk who returned to put out the flames after taking his family to a safe location. "It was scary, very scary." President Petro Poroshenko called a meeting of the heads of security forces and promised to give residents all the necessary help, his spokesman wrote on Facebook.
"The main thing is to preserve people's lives. Whatever's destroyed, we will rebuild," he wrote on Facebook.
Authorities closed the airspace in a 30-kilometre radius around the site, as well as rail and road traffic.
More than a hundred firefighters worked at the scene, while the defence ministry sent firefighting tanks.
The defence ministry denied a claim by the emergencies service that the depot contained 88,000 tonnes of ammunition, while saying the real quantity was a secret. More than half the ammunition had already been removed from the depot and taken to safer sites before the fire broke out, Tymoshenko of the General Staff said.
The SBU security service confirmed that it was looking into "possible sabotage." Ukrainian infrastructure minister Volodymyr Omelyan claimed on Facebook that for those seeking the cause of the incident: "there is only one answer: Russia-Moscow-Kremlin." Military prosecutors opened a probe into possible negligence by officials, however.
This was the sixth major fire in three years at an arms depot of the Ukrainian army. In March 2017, a huge fire set off explosions at a depot in the town of Balakliya in the Kiev-controlled part of eastern Ukraine, killing one person and injuring five others. Ukrainian authorities blamed that incident on an "act of sabotage".
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