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.

U.S. President Joe Biden said on Wednesday he had asked the Defense Department to continue its surge of weapons deliveries to Ukraine, after condemning Russia's Christmas Day attack on Ukraine's energy system and some of its cities.

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.

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

Russia declared a federal emergency on Thursday over an oil spill caused by two Russian tankers in the Black Sea, the Emergencies Ministry said.

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.