Russia's military chief of staff has said Moscow is ready to use force, including pre-emptively and with nuclear weapons, to defend itself.
Gen Baluyevsky sees a threat from countries seeking global hegemony
Gen Yuri Baluyevsky said Russia had no plans to attack anyone, but it was important that other countries clearly understood its leaders' resolve.
Correspondents say the general's remarks do not mark a shift in policy.
But they are the latest in a string of assertive comments from Moscow when relations with the West are strained.
In a speech to a military conference broadcast on state-run cable TV, Gen Baluyevsky said there were potential threats to Russia from international terrorism or countries seeking global or regional hegemony.
"We do not intend to attack anyone, but we consider it necessary for all our partners in the world community to clearly understand ... that to defend the sovereignty and territorial integrity of Russia and its allies, military forces will be used, including preventively, including with the use of nuclear weapons," he said.
President Vladimir Putin approved a new military doctrine in 2000 which stated that Russia might use nuclear weapons against nuclear or large-scale conventional attack.
Analysts say the doctrine reflects the decline in Russia's conventional forces, which can no longer be relied upon to counter aggression.
But correspondents say it is unusual for a defence official to specifically mention nuclear weapons in remarks on preventive strikes.
The remarks come days after Mr Putin appointed hardliner Dmitri Rogozin as the country's new ambassador to Nato.
Relations with the West have deteriorated in recent years, most notably with Moscow strongly opposing plans by the alliance to set up missile defence sites in Poland and the Czech Republic.
Russia has responded by suspending the Conventional Forces in Europe (CFE) Treaty.