Explosions rocked a Russian air base in Crimea Tuesday, possibly marking a dramatic escalation of the war in Ukraine. At least one person was killed and several others were wounded, authorities said.

Russia’s Defense Ministry denied the Saki base on the Black Sea had been shelled, saying munitions blew up. Ukrainian social networks, though, were abuzz with speculation that it was hit by Ukrainian-fired long-range missiles.

Videos posted on social networks showed sunbathers fleeing a nearby beach as flames and smoke rose over the horizon from multiple points, accompanied by booms. Crimea Today News said on Telegram that witnesses reported fire on a runway and damage to nearby homes as a result of what it said were dozens of explosions.

Russia’s state news agency, Tass, quoted an unidentified ministry source as saying the explosions’ primary cause appeared to be a “violation of fire safety requirements.” The ministry said no warplanes were damaged.

Ukraine’s Defense Ministry said with sarcasm on Facebook: “The Ministry of Defense of Ukraine cannot establish the cause of the fire, but once again recalls the rules of fire safety and the prohibition of smoking in unspecified places.”

If Ukrainian forces were, in fact, responsible for the blasts, it would mark the first known major attack on a Russian military site on the Crimean Peninsula, which the Kremlin annexed in 2014. A smaller explosion last month at the headquarters of Russia’s Black Sea Fleet in the Crimean port of Sevastopol, blamed on Ukrainian saboteurs using a makeshift drone, may have been a prologue to today’s escalation.

Russian warplanes have used the Saki base to strike areas in Ukraine’s south on short notice.

The regional leader of Crimea, Sergei Aksyonov, said ambulances and medical helicopters were sent to the base and the area was sealed off within a radius of three miles.

One person was killed, according to Mr. Aksyonov. Crimean health authorities said six people were wounded, one of whom remained hospitalized. 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 retaliation, including strikes on “decision-making centers” in Kyiv.

Earlier Tuesday, Ukrainian officials reported 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 more than 120 rockets at the town of Nikopol, across the Dnieper River from the plant, the Dnipropetrovsk governor, Valentyn Reznichenko, said. Several apartment buildings and industrial sites were damaged, he said.

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

In his nightly video address Monday, President Zelensky invoked the 1986 disaster at the Chernobyl nuclear plant in Ukraine. 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 is 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. The head of the Kremlin-backed administration, Evgeny Balitsky, 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,” Mr. Balitsky said.

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