"Fortification work is being carried out on the territory of Crimea under my control with the aim of guaranteeing the security of all Crimeans," the Moscow-appointed governor of the region, Sergei Aksyonov, said on social media.

Russia annexed Crimea in the wake of nationwide pro-democracy demonstrations that led to the ouster of Ukraine's former Kremlin-friendly president.

It was used as a launching pad in February for what the Kremlin calls its "special military operation" in Ukraine.

Aksyonov said that the fortification work was being carried out even though the military and law enforcement were already taking measures.

Ukraine forces in recent months have been pushing a counter-offensive in the south of the country and last week reclaimed Kherson, the capital of the Ukrainian region bordering Crimea.

Russia claimed to have also annexed that region along with three more in September, vowing to defend them with all available military means.

Ukraine President Volodymyr Zelensky has said his forces intend to recapture the peninsula as well.

There have been several explosions at or near Russian military installations in Crimea since February, including a coordinated drone attack on a key Russian naval port at Sevastopol.

In October, the Kerch bridge connecting the peninsula to the Russian mainland was partially destroyed in an attack attributed to Ukraine by Moscow.

Russia announced on Friday that it was making gains too in the eastern Donetsk region, which its forces have partially controlled since 2014.

"As a result of offensive work carried out by Russian troops, the settlement of Opytnoe was liberated," defence ministry spokesman Igor Konashenkov said on Friday.

Moscow's forces, alongside troops from two breakaway regions in east Ukraine and mercenaries from Russia's Wagner group have been trying for months to capture the nearby town of Bakhmut.

Russia has responded with a fresh barrage of missile and drone attacks across Ukraine, crippling its power grid, after its troops withdrew from Kherson.

Ukrainian President Volodymyr Zelensky said Thursday that some 10 million people were suffering blackouts as a result.

The Russian defence ministry said Friday it had launched "long-range, precision" weapons against military targets, specifically fuel and energy insfrastructure.

"The goals of the attack have been achieved. All rockets hit exactly the designated objects," Konashenkov said.