Russia says the Crimean Peninsula has been plunged into darkness, following reports of explosions at four main power lines supplying electricity from Ukraine to the disputed territory.
It wasn't clear early Sunday whether the blasts occurred in Ukrainian territory or in Crimea, which Russia annexed last year despite international outrage and a spate of Western sanctions aimed at forcing Moscow to return the peninsula to Ukrainian control.
Russia's official Tass news agency said all power to Crimea was lost shortly after midnight local time. The agency said neither police nor Crimean authorities had identified the cause of the blackout.
Ukraine's UNIAN news agency reported Friday that two of the four main power lines had been disabled following an explosion of an unidentified device.
By Saturday, Tass was quoting the director of Krymenergo, the Crimea energy company, as saying, "Crimea is completely cut off," but there was no comment on the situation from Kyiv.
Separately, Ukraine's Channel 5 television reported that Ukrainian police had surrounded a group of Tatar activists near the downed lines. The report, quoting Lenur Islyamov, the head of a group advocating the blockade of Crimea, described the situation at the site of the downed power lines as "tense."
The Russian parliament voted overwhelmingly in March 2014 to annex the largely Russian-speaking territory, just weeks after pro-Western Ukrainian protests in Kyiv forced Russia-leaning President Viktor Yanukovych from office.
Weeks later, pro-Russian separatists in eastern Ukraine launched a rebellion against Kyiv's rule that triggered a series of trade and travel sanctions against key Russian officials close to President Vladimir Putin.
Those sanctions are due to expire in January. But the Reuters news agency, reporting Saturday, said Western leaders meeting on the sidelines of a G-20 summit in Turkey had agreed to extend the sanctions until July 2016.
The United Nations said in September that nearly 8,000 people, most of them civilians, have been killed in the Ukrainian conflict.