Russian Foreign Minister Sergei Lavrov said on Wednesday that if a third World War were to take place, it would involve nuclear weapons and be destructive, the RIA news agency reported.
Lavrov has said that Russia, which launched what it calls a special military operation against Ukraine last week, would face a "real danger" if Kyiv acquired nuclear weapons.
A week after launching its invasion of Ukraine, Russia said its forces took control the first sizable city on Wednesday, seizing Kherson, in the south, as fighting raged around the country and Western nations tightened an economic noose around Russia.
More than half a million Ukrainians have fled the fighting since Russian President Vladimir Putin ordered a full-scale invasion of his southern neighbour nearly a week ago.
Russian bombardments of Ukrainian cities continued, with video posted on social media showing heavily damaged buildings around the second city of Kharkiv.
On Tuesday, Lavrov told a Geneva disarmament meeting that Ukraine has been seeking to acquire nuclear weapons, a "real danger" that needed a Russian response.
Read more: Russia says 'real danger' of Ukraine acquiring nuclear weapons required response
"Today the dangers that (Ukrainian President Volodymyr) Zelenskiy's regime pose for neighbouring countries and international security in general have increased substantially after the authorities set up in Kyiv have embarked upon dangerous games related to plans to acquire their own nuclear weapons," the Russian foreign minister told the Conference on Disarmament in a video address.
"Ukraine still has Soviet nuclear technologies and the means of delivery of such weapons. We cannot fail to respond to this real danger," he said, also calling for Washington to rebase its nuclear weapons from Europe.
He delivered the speech to a thin crowd after many diplomats including France and Britain staged a walk-out to protest Russia's invasion of Ukraine as they did at a parallel UN meeting in Geneva on Tuesday.