Russia's Defense Ministry said that munitions blew up at the Saki base, and it emphasized that the installation had not been shelled. But Ukrainian social networks were abuzz with speculation that it was hit by Ukrainian-fired long-range missiles.

There was no immediate comment from Ukrainian authorities.

Videos posted on social networks showed sunbathers fleeing a nearby beach as huge clouds of smoke from the explosions rose over the horizon.

It the base was, in fact, struck by the Ukrainians, it would mark the first known major attack on a Russian military site on the Crimean Peninsula, which was annexed by the Kremlin in 2014. The headquarters of Russia's Black Sea Fleet in the Crimean port of Sevastopol was hit by a small-scale explosion delivered by a makeshift drone last month in an attack blamed on Ukrainian saboteurs.

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 five kilometers (three miles).

Five people were wounded, and one of them was hospitalized, said Konstantin Skorupsky, the head of Crimea's health care department. He said the others were treated for cuts from shards of glass and were released.

Officials in Moscow have long warned Ukraine that any attack on Crimea would trigger massive retaliation, including strikes on “decision-making centers” in Kyiv.

The Saki base was used by Russian warplanes to strike areas in Ukraine's south on short notice.

Earlier Tuesday, Ukrainian officials that at least three Ukrainian civilians were killed and 23 wounded by Russian shelling in 24 hours, including an attack not far from the Russian-occupied Zaporizhzhia nuclear power plant.

The Russians fired over 120 rockets at the town of Nikopol, which is across the Dnieper River from the plant, Dnipropetrovsk Gov. Valentyn Reznichenko said. Several apartment buildings and industrial sites were damaged, he said.

Ukraine and Russia have accused each other in recent days of shelling the power station, the biggest nuclear plant in Europe, stoking international fears of a catastrophe.

In his nightly video address, Ukrainian President Volodymyr Zelenskyy invoked the 1986 disaster at the Chernobyl nuclear plant in Ukraine, which at the time was a Soviet republic. He called for new sanctions against Russia, accusing it of risking another nuclear disaster.

“We are actively informing the world about Russian nuclear blackmail," he said.

The Kremlin claimed that Ukraine's military was attacking the plant and urged Western powers to force Kyiv to stop.

A Russian-installed official in the partially occupied Zaporizhzhia region said an air defense system at the plant would be reinforced in the aftermath of last week's shelling. Evgeny Balitsky, head of the Kremlin-backed administration, told Russian state TV that power lines and other damaged portions of the plant were restored.

“The plant is operating normally but, of course, with an increased degree of security,” Balitsky said.

The Ukrainians in recent weeks have been mounting counterattacks in Russian-occupied areas of southern Ukraine while also trying to hold off the Kremlin's forces in the country's industrial Donbas region in the east.?