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.

"Ukraine was brutally attacked on Christmas Day," said Keith Kellogg, President-elect Donald Trump's pick for Ukraine and Russia envoy.

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.

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.

Finnish police are investigating whether a Russian ship was involved in the sabotage of an electricity cable running between Finland and Estonia. The authorities said on Thursday that they believe the anchor of the Eagle S, a tanker registered with the Cook Islands, may have damaged the Estlink 2 cable, which became disconnected on Wednesday.

Damage to the power cable in the Baltic Sea is the latest in a spate of murky disruptions to undersea cables that have put European authorities on high alert.

Police and media reports say that Finnish authorities have detained a Russia-linked ship as they investigate whether it damaged a Baltic Sea power cable and several data cables