STORY: 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.
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 U.N. General Assembly.