Blasts have smashed a Russian air base near seaside resorts in the annexed Crimean peninsula, killing one person and injuring five, according to local authorities.
Local witnesses told reporters they heard at least 12 explosions around 3: 20 pm local time 1220 GMT from Saky air base near Novofedorivka on Crimea's western coast.
They described a final blast as the loudest around 30 minutes later.
Crimea has so far been spared from the intense bombardment and artillery combat that have taken place in other areas of eastern and southern Ukraine since February 24, when President Vladimir Putin ordered Russian armed forces into Ukraine, including some based in the peninsula.
Russia's defence ministry said the detonation of several aviation ammunition stores had caused an explosion.
According to the state-owned news agency TASS, five people have been injured, including one child.
Sergei Aksyonov, the head of Crimea's administration, announced his death.
There was no immediate response from Ukraine.
Moscow officials have warned Ukraine that a strike on Crimea would cause massive retaliation, including strikes on decision-making centres in Kyiv.
A five-kilometer exclusion zone had been established around the site, and local authorities were monitoring fires, according to Aksyonov.
The Defence Ministry said there had not been any kind of attack and that no aviation equipment had been damaged.
Moscow said further measures were being taken to establish the causes.
The base is close to the seaside resorts of Novofedorivka and Saky, but the Russian tourism association ATOS said they did not appear to have been affected by the blasts.
Locals reported congestion on roads leading away from the coast.
Numerous videos taken from beaches were posted on social media, showing huge plumes of smoke rising into the sky in the distance.
The videos could not be verified immediately.
Novofedorivka and Saky are located about 50 km north of the port of Sevastopol, the home of Russia's Black Sea Fleet, which Moscow leased from Kyiv for decades before annexing the entire peninsula from Ukraine in 2014, a move not recognized by most other countries.