STORY: At a meeting of the United Nations Security focused on the bitter and bloody conflict in Ukraine, U.N. Secretary General Antonio Gutteres warned Russia that plans to annex Ukrainian territory under the guise of referendums in Russian-held areas would violate the U.N. charter.
"Any annexation of the state's territory by another state, resulting from the threat or use of force is a violation of the U.N. Charter and of international law."
Russia said it plans to hold referendums in the coming days in parts of Ukraine it seized since the start of its invasion some seven months ago. Those territories comprise almost 15 percent of Ukraine.
On Wednesday, Russian president Vladimir Putin announced plans to call up some 300,000 reservists for combat, and made a thinly-veiled threat to use nuclear weapons to defend the country.
Gutteres said nuclear threats were unacceptable.
"The idea of nuclear conflict - once unthinkable - has become subject of debate. This, in itself, is totally unacceptable."
Much of the Security Council meeting Thursday focused on alleged atrocities committed in Ukraine.
"When I went to Bucha, and went behind St. Andrews Church, the bodies I saw were not fake."
International Criminal Court 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.
U.S. Secretary of State Antony Blinken told the Security Council of a mass grave where victims showed signs of torture.
"One site contained some 440 unmarked graves. A number of the bodies unearthed so far reportedly showed signs of torture, including one victim with broken arms and a rope around his neck."
Russia denies committing war crimes as part of what it terms a "special military operation."
On Thursday, Russian foreign minister Sergei Lavrov defended the his country's military actions, claiming that Ukraine, in fact, had sparked the conflict by threatening Russia, and repeating Moscow's justifications for sending their forces into their smaller neighbor.
"Everything I've said today simply confirms that the decision to conduct the special military operation was inevitable."
Lavrov left after speaking.
Ukrainian foreign minister Dmytro Kuleba spoke to reporters ahead of the security council meeting.
When asked about being in the same room as Lavrov, he responded:
"I will keep safe social distance with him."