Ukraine said on Thursday it suspects that Russia's military or separatists are behind blowing up a warehouse storing tank ammunition at a military base in eastern Balaklia city.
Nobody was hurt, but some 20,000 people have been evacuated following several explosions in the city, about 100 kilometres from the frontline of Ukraine's war against Russian-backed rebels.
"We have a 'friendly' country - the Russian Federation," Ukrainian Defence Minister Stepan Poltorak said.
I think that first of all it could be representatives who help the (separatist) groups that carry out combat missions.
Ukraine did not provide evidence of Russian or rebel involvement.
Earlier, the Ukrainian military said unknown saboteurs blew up the warehouse.
"According to preliminary data ... as a result of sabotage, last night at 2:46 AM local time (0046 GMT), fire and explosions caused the detonation of ammunition at several sites storing rockets and artillery weapons," Ukraine's chief military prosecutor Anatoly Matios wrote on Facebook.
The base contained about 138,000 tonnes of ammunition.
Rescue teams were evacuating nearby villages in the eastern Kharkiv region, the military said.
Military spokesman Oleksander Motuzyanyk said security around other bases was being beefed up.
Ukrainian Prime Minister Volodymyr Groysman was due to fly to the area later on Thursday.
Saboteurs previously tried to destroy the same base using drones in 2015, another military spokesman, Yuzef Venskovich, told the 112 TV channel.
TRT World spoke to Volodymyr Solohub for the latest on the blasts.
Base still burning
Poltorak said a third of the base is still burning and the airspace above it was closed off, adding that the attack would not significantly affect Ukraine's military capacity.
More than 10,000 people have been killed in the Ukraine conflict in the past three years.
A ceasefire agreed in Minsk in 2015 has been routinely violated.