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.

"No one claims that it was done on purpose," an Azerbaijan official told Reuters. They added their government "expects the Russian side to confess."

A U.S. official tells NBC News there are early indications that the passenger plane that crashed in Kazakhstan may have been hit by a Russian anti-aircraft system. The official said if it turns out to be the case,

Azerbaijani and US officials believe a Russian surface-to-air missile caused the deadly crash of an Azerbaijani passenger jet, media reports and a US official said Thursday, as the Kremlin cautioned against "