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.