Antonio Guterres branded Russia's nuclear threats against the West as being "totally unacceptable" during the United Nations General Assembly in New York.
The UN secretary-general warned world leaders that the latest developments in the war in Ukraine are "dangerous and disturbing," and a step closer to an "endless cycle of horror and bloodshed."
"The idea of nuclear conflict, once unthinkable, has become a subject of debate. This in itself is totally unacceptable," he said.
Mr Guterres also deemed the idea of any referendum being held in occupied parts of Ukraine as a "violation of international law."
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