A Ukrainian serviceman walks on a street at the frontline in Mykolaiv region, Ukraine, on Monday. Evgeniy Maloletka/Associated Press

KYIV, Ukraine — Powerful explosions rocked a Russian air base in Crimea and sent towering clouds of smoke over the landscape Tuesday in what may mark an 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 and said instead that munitions had blown up there. But Ukrainian social networks 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 huge flames and pillars of smoke rose over the horizon from multiple points, accompanied by loud 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 blasts.

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 sarcastically 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.”

During the war, Russia has reported numerous fires and explosions at munitions storage sites on its territory near the Ukrainian border, blaming some of them on Ukrainian strikes. Ukrainian authorities have mostly remained mum about the incidents.

If Ukrainian forces were, in fact, responsible for the blasts at the air base, 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 was blamed on Ukrainian saboteurs using a makeshift drone.

A religious icon is seen from the window of a church which was destroyed after Russian attack at the frontline in Mykolaiv region, Ukraine, on Monday. Evgeniy Maloletka/Associated Press

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

Crimea’s regional leader Sergei Aksyonov said ambulances and medical helicopters were sent to the base and the area was sealed off within a radius of five kilometers (three miles).

One person was killed, Aksyonov said. Crimean health authorities said nine people were wounded, one of whom remained hospitalized. Others were treated for cuts from shards of glass and 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.

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 over 120 rockets at the town of Nikopol, 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 of shelling the power station, Europe’s biggest nuclear plant, stoking international fears of a catastrophe.

The governor of the region where the plant is situated, Oleksandr Starukh, said Tuesday that radiation levels were normal. But he warned that an accident could spread radiation whichever way the wind blows, carrying it to Moscow and other Russian cities.

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 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 industrial Donbas region in the east.