STORY: Since Moscow invaded its neighbor on Feb. 24 last year, Russian President Vladimir Putin has repeatedly hinted that Russia could use a nuclear weapon if threatened.

"We have heard implicit threats to use nuclear weapons. The so-called tactical use of nuclear weapons is utterly unacceptable. It is high time to step back from the brink," Guterres told the 193-member U.N. General Assembly at a meeting to mark the first anniversary of the war in Ukraine.

The General Assembly is set to adopt a draft resolution - likely on Thursday - stressing "the need to reach, as soon as possible, a comprehensive, just and lasting peace" in line with the U.N. Charter.

The United States and western allies have called the invasion an unprovoked land grab against a sovereign nation.