Russia's security policy dictates that the country would only use nuclear weapons if its very existence were threatened, Kremlin spokesman Dmitry Peskov told CNN in an interview on Tuesday.
The comment, nearly four weeks after Russia sent its forces into Ukraine, came amid Western concern that the conflict there could escalate into a nuclear war.
Peskov made the comment in an English-language interview when asked whether he was confident President Vladimir Putin would not use nuclear weapons.
"We have a concept of domestic security and it's public, you can read all the reasons for nuclear arms to be used. So if it is an existential threat for our country, then it (the nuclear arsenal) can be used in accordance with our concept," he said.
"There are no other reasons that were mentioned in that text," he said in a further reference to the country's security concept.
Putin last month ordered Russia's nuclear forces to be put on high alert. In line with the order, Russia's defence ministry said on Feb. 28 that its nuclear missile forces and Northern and Pacific fleets had been placed on enhanced combat duty, the Interfax news agency reported.
United Nations Secretary-General Antonio Guterres said on March 14: "The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility."
Intense Russian airstrikes are turning besieged Mariupol into the "ashes of a dead land", the city council said on Tuesday, as the United States and Europe planned more sanctions to punish Moscow for its invasion of Ukraine.
Street fighting and bombardments raged in Mariupol, Ukrainian officials said, a day after it rejected an ultimatum from Russia to surrender. Hundreds of thousands are believed to be trapped inside buildings, with no access to food, water, power or heat.
Russian forces and Russian-backed separatist units had taken about half of the port city, normally home to around 400,000 people, Russia's RIA news agency said, citing a separatist leader.
Street fighting was taking place in the city, and both civilians and Ukrainian troops were coming under Russian fire, said regional governor Pavlo Kyrylenko.