Former Russian President Dmitry Medvedev said on Monday that it remained a mystery who was behind a wave of drone disruption in various European countries, but that the incidents served as a useful reminder to Europeans of the dangers of war.

Russia has accused the U.K. of plotting a sabotage operation in a move that shows Moscow is setting the conditions for a conflict with NATO, according to the Institute for the Study of War (ISW).

NATO fighter jets shot down multiple Russian drones that violated Polish airspace during an attack on neighboring Ukraine early on Wednesday, as the military alliance denounced Moscow for “absolutely dangerous” behavior that ratcheted up tensions to a new level.

Estonia labels the airspace intrusion as a "dangerous escalation," adding that "Russia's dangerous behavior cannot be tolerated". Russia and Ukraine accused each other on Monday of launching deadly drone attacks in civilian areas of their countries.

Russian air defenses intercepted several Ukrainian drones heading toward the capital Moscow early Tuesday, in the second such incident within 24 hours. Moscow Mayor Sergei

Russia struck two passenger trains in quick succession, first targeting a local service and then one bound for the capital, Kyiv, officials said.

President Vladimir V. Putin lashed out at “European elites” for “whipping up the hysteria” about the “Russian threat.”

Romania has condemned Moscow’s “irresponsible actions” after alleging that a Russian drone breached Romanian airspace during an attack on neighboring Ukraine, prompting Bucharest to scramble fighter jets.

The warning is part of a concerted Russian effort to deter President Donald Trump from giving Ukraine access to the missiles, repeating a tactic Moscow has used throughout the war.