This video from social media shows a mushroom cloud of thick smoke rising in Crimea, where authorities say one person was killed in an attack on a Russian air base.
Witnesses said they heard explosions near Saky Air Base in the annexed Crimean Peninsula.
Officials say some people were wounded by the powerful blasts, which Moscow claims were just part of a Russian operation to blow up ammunition stores.
Crimea's head Sergei Aksyonov said ambulances and medical helicopters were sent to the Saki Air Base and the area was sealed off within a radius of 5km (three miles).
The base was used by Russian warplanes to strike areas in Ukraine's south on short notice.
There was no immediate comment from Ukrainian authorities, amid speculation the damage could have been caused by long-range Ukrainian missiles.
If the base was struck by the Ukrainians, it would mark the first known major attack on a Russian military site on the Crimean Peninsula – annexed by the Kremlin in 2014 – and a significant escalation of the conflict.
Officials in Moscow have long warned Ukraine that any attack on Crimea would trigger massive retaliation, including strikes on "decision-making centres" in Kyiv.