Russia is conducting some of the biggest nuclear exercises in years, involving 64,000 people to drill its forces in "the preparation and use of nuclear forces in the event of aggression".
Russian President Vladimir Putin told Belarusian President Alexander Lukashenko, Russian Defence Minister Andrei Belousov and top generals the use of such weapons would always be an exceptional and extreme measure of last resort.
"Given the growing tensions in the world and the emergence of new threats and risks, our nuclear triad must continue to serve as a reliable guarantor of the sovereignty of the Union State of Russia and Belarus," Putin said in the Kremlin.
While Russia does not want to get involved in an arms race, he added, it will develop its nuclear forces and keep them on a sufficient level, including with new missiles and submarines.
Russia has the world's biggest nuclear arsenal with about 4400 deployed and stockpiled nuclear warheads, while the US has about 3700, according to the Federation of American Scientists.
China is the world's third-largest nuclear power with about 620, followed by France with 290 and Britain with about 225, the federation says.
Russia and China say US President Donald Trump's Golden Dome missile-defence shield plans threatened strategic stability.
As part of the drills, Russia test-fired a Yars intercontinental ballistic missile from the Plesetsk Cosmodrome in northern Russia and a Zircon hypersonic missile from a frigate in the Barents Sea, while a submarine launched a liquid-fuelled Sineva ballistic missile, the defence ministry said.
Russia displayed a Borei-class nuclear-powered ballistic missile submarine, Il-38 anti-submarine aircraft and a MiG-31 armed with a Kinzhal hypersonic missile.
Units in Belarus and Russia were issued with nuclear munitions as part of the exercises, Valery Gerasimov, chief of the Russian General Staff, told Putin.
Russian nuclear exercises typically use dummy warheads. One video released by the defence ministry showed a tarp-backed military truck travelling with minimal security.
The joint drills were met with concern from Ukraine and its NATO allies. Kyiv has repeatedly accused Moscow of planning to launch a new attack from Belarus.
Ukraine's Security Service (SBU) said its units and the army were "carrying out a comprehensive set of enhanced security measures in the northern regions of our country" bordering Belarus.
The nuclear drills come as Moscow says it is locked in an existential struggle with the West over Ukraine. Throughout the war, Putin has issued reminders of Russia's nuclear might as a warning to the West not to go too far in its support of Kyiv. Ukraine and some Western leaders have dismissed such moves as irresponsible sabre-rattling.
Moscow has accused the Baltic countries of allowing Ukraine to fly over their territory to attack northern Russia, an accusation NATO denies. The Baltic states, all strong backers of Ukraine, say Russia is redirecting Ukrainian drones into their airspace from their intended targets in Russia.
The Kremlin criticised remarks by Lithuania's top diplomat as "verging on insanity" after Foreign Minister Kestutis Budrys said NATO had to show Moscow it was capable of penetrating the Russian exclave of Kaliningrad.
Kaliningrad is sandwiched between NATO members Lithuania and Poland on the Baltic coast.
