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 contrasted its nuclear strategy to 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 plan to claim victory within days of invading Ukraine had failed, Putin admitted the war could be a "lengthy process".
However, he said 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 17th and 18th Century ruler before.
But - despite claiming the regions of Kherson, Zaporizhzhia, Luhansk and Donetsk as Russia's new territory - Moscow does not fully control any of those areas.
Last month, Russian forces were forced to retreat from Kherson city, the only regional capital they had seized since the February invasion.