Russia's foreign minister told a Geneva disarmament meeting on Tuesday that Ukraine has been seeking to acquire nuclear weapons, a "real danger" that needed a Russian response.
Russia invaded Ukraine on Thursday in what it called a special operation to demilitarise and "denazify" the country - a justification dismissed by Kyiv and the West as propaganda.
"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," Russian Foreign Minister Sergei Lavrov told the Conference on Disarmament in a video address.
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"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 U.N. meeting in Geneva on Tuesday.
They stood in a circle outside the meeting for the duration of Lavrov's speech, holding a Ukrainian flag. Lavrov was supposed to attend the session in person but the visit was cancelled, with Russia accusing unidentified EU states of blocking his flight path.
Earlier, a minute of silence was held for the victims of fighting in Ukraine.
Russia's invasion of Ukraine has enlivened the deadlocked arms negotiating forum that has failed to reach a deal in decades despite holding over 1,600 plenary meetings.
At the same meeting, Ukraine's foreign minister accused Russia of war crimes through its shelling of his country and the ambassador called for a special meeting to address Russian aggression and weapons of mass destruction.
A copy of the request seen by Reuters said the meeting would address the "the use of conventional weapons against the civilian population as well as nuclear and other WMD threats" and called for concrete and specific actions.
It said the request was supported by "numerous other" members of the 65-member body, without naming them. Russia's delegate objected to the meeting.