Russian President Putin last month proclaimed that the territories Moscow was taking from Ukraine would be part of Russia "for ever". But Russia does not wholly control any of the four regions and has yet to define their borders.
Russia has said that four Ukrainian regions whose annexation it proclaimed last month are under the protection of its nuclear arsenal.
The statement from the Kremlin came on Tuesday at a moment of acute tension, with both NATO and Russia expected to hold military exercises shortly to test the readiness of their nuclear weapons forces.
Asked by reporters if the regions were under Moscow's nuclear umbrella, Kremlin spokesperson Dmitry Peskov said: "All these territories are inalienable parts of the Russian Federation and they are all protected. Their security is provided for at the same level as [it is for] the rest of Russia's territory."
President Vladimir Putin said last month that Moscow was ready to use nuclear weapons if necessary to defend Russia's "territorial integrity".
US President Joe Biden said on October 6 that his threat had brought the world closer to "Armageddon" than at any time since the 1962 Cuban Missile Crisis, when many feared a nuclear war might be imminent.
READ MORE: UN condemns Russia's 'illegal annexation' of Ukraine territories
Nuclear preparedness exercises
Nearly eight months into his military offensive against Ukraine, some analysts believe the likelihood of Putin resorting to nuclear weapons has increased since his army suffered a series of major defeats.
Putin's ally Alexander Lukashenko, the president of Belarus, warned the West last week against backing Russia into a corner.
Other analysts have argued the nuclear risk is overstated, suggesting it would be suicidal for Putin to embark on such an escalation.
NATO is conducting nuclear preparedness exercises this week and has said it expects Russia to hold its own nuclear drills imminently, but Peskov said he had no information on that.
"There is an established system of notifications to inform about the conduct of exercises, and this is carried out through the channels of the Ministry of Defence," he said.
READ MORE: Biden: Nuclear 'Armageddon' risk highest since Cuban Missiles Crisis
Uncertainty in annexed territorries
Putin last month proclaimed that the territories Moscow was taking from Ukraine would be part of Russia "for ever". But Russia does not wholly control any of the four regions and has yet to define their borders.
The annexation was condemned as illegal by Ukraine, its allies and an overwhelming majority of countries in the United Nations General Assembly.
Russia has lost ground in the four regions even since it claimed control over them in a lavish Kremlin ceremony on September 30.
Peskov said work was still under way to integrate them into Russia's legal, economic and security systems.
READ MORE: Putin ally says Russia's nuclear weapons can be used to defend new regions