Russia issues nuclear warning to NATO as tensions rise in the Baltic

"Russia will be ready to use the entire arsenal of forces and capabilities at its disposal, including nuclear weapons, to defend its territory should NATO countries undertake any attempt aimed at isolating the Kaliningrad Region from the rest of the country," said the text, classed as a "non-paper" — diplomatic jargon for an informal document.

In addition to the private warning, Russian embassies in at least three European countries have issued statements in the past three days accusing NATO of escalating tensions through military exercises and planning scenarios involving Kaliningrad. Russia also rejected Western allegations that it poses a threat to Europe.

Read:EU foreign policy chief claims Russia planning 'more sabotage acts' against Europe. The statements said Russia had no intention of attacking NATO countries but warned that any aggression against Kaliningrad — a heavily militarised Russian territory that is sandwiched between Poland and Lithuania — would be met with the strongest possible response.

The warnings highlight a sharp rise in tension between Russia and European countries, which accuse Moscow of stepping up a "hybrid war" on the continent through acts including sabotage and arson. Russia denies that and rejects repeated statements by Western politicians that it is gearing up for a possible military attack on a NATO country.

NATO denounces 'irresponsible nuclear rhetoric'NATO spokesperson Allison Hart confirmed Russia had sent a non-paper to the alliance but did not disclose its contents. "We have sent a response, noting that NATO is a defensive Alliance and none of our activities or exercises pose a risk to any part of Russia.

We strongly denounce the threat of force, including any irresponsible nuclear rhetoric. We call on Russia to end its unprovoked war in Ukraine," she said. Read more:Is NATO ready to fight?

The Ukraine war is raising doubts among military analystsA senior European defence official said: "(This is the) first time since (the) Cold War that Russia is threatening us like this. So this is serious.

"A NATO diplomat said: "I would say that the Russian letter is only another Russian attempt to intimidate Europeans, and that the response by NATO is firm and responsible. (It) reaffirms that we will not be dissuaded from supporting Ukraine.

"Kremlin says Kaliningrad nuclear warnings meant as reminder to European "hotheads"The Kremlin said on Wednesday that Russia issued nuclear warnings to NATO over the Russian exclave of Kaliningrad to remind "hotheads" in Europe of the provisions of its existing policy.

Asked to comment, Kremlin spokesman Dmitry Peskov said: "It is only natural that our diplomats are trying to remind hotheads in Europe of our policy documents on this matter. "