Russia's top security agency says it has arrested four Russians accused of making preparations to kill senior Defense Ministry officials on Ukraine's orders.

The Federal Security Service of Russia said a series of assassination attempts had been foiled. Four Russian citizens have been arrested.

Ukrainian President Volodymyr Zelenskiy said on Monday that more than 3,000 North Korean soldiers have been killed and wounded in Russia's Kursk region and warned that Pyongyang could send more personnel and equipment for Moscow's army.

Some 3,700 tons of low-grade fuel oil have spilled into the Black Sea since two Russian oil tankers were severely damaged in a storm on Dec. 15.

Crimean activists warn that the situation on the occupied peninsula could be even worse than on the Russian side of the Black Sea coast. According to one of them, the consequences of the oil spill on the Crimean beaches are more dangerous than in the Russian city of Anapa.

Russia launched aerial attacks on Ukraine’s energy sector “on a massive scale” overnight into Wednesday, local authorities said, with explosions reported across the country amid intensified bombardments that have left Ukraine in a precarious position while the war grinds into a third winter.

Russia attacked Ukraine's energy system and some cities in an "inhuman" Christmas Day assault, Ukraine's President Volodymyr Zelenskyy said.

Ukraine's air force says it detected 184 missiles and drones, causing damage to energy infrastructure across the country.