Vladimir Putin has said the threat of a nuclear war was rising, but insisted Russia had not "gone mad" and would not use its nuclear weapons first.
The Russian president insisted that his country would only use weapons of mass destruction in response to an attack.
Speaking at Russia's annual human rights council meeting, he also said the war in Ukraine could be a "lengthy process".
Western officials believe Putin initially planned for a rapid victory.
Russia's capacity to use nuclear weapons has come under increased scrutiny since it invaded Ukraine in February.
"Such a threat is growing, it would be wrong to hide it," Putin warned while talking about the prospect of nuclear war via video link from Moscow.
But he asserted that Russia would "under no circumstances" use the weapons first, and would not threaten anyone with its nuclear arsenal.
"We have not gone mad, we are aware of what nuclear weapons are," he said, adding: "We aren't about to run around the world brandishing this weapon like a razor."
Putin also boasted that Russia had the most modern and advanced nuclear weapons in the world, and compared Russia's nuclear strategy to that of the US, who he said had gone further than Russia by locating its nuclear weapons on other territories.
"We do not have nuclear weapons, including tactical ones, on the territory of other countries, but the Americans do - in Turkey, and in a number of other European countries," he said.
Putin has previously insisted that Russia's nuclear doctrine only allowed for the defensive use of nuclear arms.
Appearing to recognise that his initial plans to claim victory within days of invading Ukraine had failed, Putin admitted that the war could be a "lengthy process".
However, he said that the results had already been "significant", for example the new territories Russia has illegally claimed after sham-referendums in four regions of Ukraine.
He boasted that the annexations had made the Sea of Azov - which is bordered by south-east Ukraine and south-west Russia - an "internal sea" of Russia, adding that this was an aspiration of Russian Tsar Peter the Great. President Putin has compared himself to the ruler before.
In the run-up to Wednesday's meeting, 10 members of the council who had expressed doubts about the war were removed. Pro-war replacements were brought in instead.
Subjects to be discussed during the meeting were also heavily vetted beforehand, according to the independent Russian news outlet Vertska.
In recent weeks, Russia's nuclear doctrine has come under close scrutiny on the circumstances in which it could use nuclear arms, in particular a "tactical" weapon that might be unleashed on the battlefield in Ukraine.
A tactical nuclear weapon is for use in combat, as opposed to the larger "strategic" weapons which are designed to cause massive destruction.
You may also be interested in: