The Russian Foreign Ministry on Sunday warned Ukraine of "irreversible consequences" after a cross-border shelling, stoking concerns of a nuclear threat.
The ministry said it had officially protested the shelling, which killed one Russian man. It described the event as "an aggressive act by the Ukrainian side against sovereign Russian territory and the citizens of the Russian Federation," Reuters reported.
"This represents a qualitative escalation of the danger to our citizens, now even on our own territory," Russian Deputy Foreign Minister Grigory Karasin said in an interview with state television. "Of course this naturally cannot pass without a response."
This latest statement comes on the heels of Russian Foreign Minister Sergei Lavrov's comments last week, which seemed to constitute a possible nuclear warning to Kiev should it ever try to militarily retake Crimea, according to The Diplomat.
"If it comes to aggression against Russian territory, which Crimea and Sevastopol are parts of, I would not advise anyone to do this," Lavrov said during a press briefing. "We have the doctrine of national security, and it very clearly regulates the actions, which will be taken in this case."
The minister was responding to recent statements made by senior Ukrainian officials promising to retake Crimea. Russia's current military doctrine authorizes the use of nuclear force against a large-scale conventional attack by regional foes.
The doctrine, revised in 2010, states: "The Russian Federation reserves the right to utilize nuclear weapons ... in the event of aggression against the Russian Federation involving the use of conventional weapons when the very existence of the state is under threat."
The Diplomat noted that, at the press conference, Lavrov referred to this military doctrine and added that the Kremlin views Crimea as an important part of Russian territory.
Russian President Vladimir Putin lately has appeared to signal that Russia would dial back its military involvement in Ukraine's struggles with separatists, many of whom are assumed to be sponsored by Moscow. However, the Kremlin's recent rhetoric could indicate that the nuclear superpower is not done intervening with force in its former Soviet satellite state, months after it unilaterally annexed the Crimean Peninsula, according to Reuters.
Meanwhile, the Ukrainian government denied that its armed forces were behind the recent shelling, calling it "total nonsense" and potentially the work of separatists.