UN Secretary-General Antonio Guterres told a Security Council meeting on Ukraine on Thursday that talk of a nuclear conflict is "totally unacceptable" and effectively warned that so-called referenda in areas under Russian control would be annexation.
Referendums on joining Russia are due to take place from Friday until Tuesday in several largely Russian-held regions in eastern and southern Ukraine, which comprise around 15% of the country's territory.
Guterres told the council ministerial meeting that he was concerned about the plans for "so-called 'referenda'." Russian Foreign Minister Sergei Lavrov is due to address the 15-member council, but was not in the chamber for Guterres' remarks.
"Any annexation of a state's territory by another state resulting from the threat or use of force is a violation of the UN Charter and of international law," Guterres said.
Russian President Vladimir Putin on Wednesday threatened to use nuclear weapons to defend Russia.
The Security Council meeting on atrocities committed in Ukraine is taking place during the annual gathering of world leaders for the UN General Assembly. Russia invaded Ukraine on Feb. 24 as the Security Council met in New York to discuss Western concerns about such a move by Moscow.
International Criminal Court (ICC) Prosecutor Karim Khan told the council that there were "reasonable grounds" to believe crimes within the jurisdiction of the court had been committed in Ukraine. The Hague-based court handles war crimes, crimes against humanity, genocide and crimes of aggression.
Khan said the ICC investigation priorities were intentional targeting of civilian objects and the transfer of populations from Ukraine, including children.
The United States has said estimates from a variety of sources, including Moscow, indicate that authorities have "interrogated, detained, and forcibly deported" up to 1.6 million Ukrainians to Russia since Moscow's invasion.
"The very international order we've gathered here to uphold is being shredded before our eyes. We can't let President Putin get away with it," U.S. Secretary of State Antony Blinken told the council. Lavrov was not in the chamber to hear his remarks.
Ukraine, the United States and others have accused Russia of war crimes in Ukraine. Russia denies targeting civilians during what it calls its "special military operation," describing accusations of human rights abuses as a smear campaign.
Asked if he might speak with Lavrov at the Security Council on Thursday, Ukraine's Foreign Minister Dmytro Kuleba said he will "keep safe social distance."
The council has been unable to take any meaningful action on Ukraine because Russia is a permanent veto-wielding member along with the United States, France, Britain and China. The meeting on Thursday will be at least the 20th time the Security Council has met on Ukraine this year.
Ukraine's chief war crimes prosecutor told Reuters last month his office is investigating almost 26,000 suspected war crimes cases committed since Russia's Feb. 24 invasion and has charged 135 people.