UN Secretary General Antonio Guterres said on Wednesday that Russia's invasion of Ukraine breached the UN Charter and international law.
Mr Guterres was speaking a day before the UN General Assembly is set to vote on a new resolution calling for peace.
“All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations,” he said.
He reaffirmed the UN's commitment to the “sovereignty, independence, unity and territorial integrity of Ukraine, within its internationally recognised borders”.
Mr Guterres said the General Assembly had addressed, in the clearest terms, the attempt by the Russian Federation to illegally annex four regions of Ukraine — Kherson, Zaporizhzhia, Donetsk and Luhansk.
He said “implicit threats” to use nuclear weapons were “utterly unacceptable”.
“Every day, we are reminded of the grave threat that haunts us all when irresponsible military activity continues around the Zaporizhzhia Nuclear Power Plant, the largest nuclear facility in Europe,” Mr Guterres said.
Russia's President, Vladimir Putin, said in his annual address to parliament on Tuesday that he would be suspending his country's participation in the last remaining arms control treaty with the US, known as the New Start treaty.
Days before the February 24 anniversary of Russia's invasion of Ukraine, he also threatened to resume testing of nuclear weapons.
Ukraine's Foreign Minister, Dmitryo Kuleba, urged the 193-member UN General Assembly to vote on a non-legally binding resolution stressing “the need to reach, as soon as possible, a comprehensive, just and lasting peace” in line with the founding UN Charter.
The resolution which was drafted by Ukraine in consultation with its partners makes no direct reference to President Zelensky’s 10 point peace plan presented at the G20 summit in November.
The goal was to make it easy to back the text.
Mr Kuleba said he had a message for member states that do not want to take sides and seek an end to the war “with whatever result.”
"In this war there are not two sides," he said, “there is an aggressor and a victim.”
“We never wanted war … we know what we are fighting for. We are defending our land, our families and our homes.”
“Can anyone tell us what the Russians are dying for?” he asked.
The resolution demands a “cessation of hostilities” and the withdrawal of Russia’s military forces from Ukrainian territory “within its internationally recognised borders”.
It reaffirms that no territorial acquisition “resulting from the threat or use of force shall be recognised as legal".
Russia's UN ambassador Vasily Nebenzya said on Wednesday Western states “ignored” Moscow's concerns and “continue bringing the military infrastructure of Nato closer and closer” to his country's borders.
Mr Nebenzya told the General Assembly, Moscow “had no other option” but to launch a “special military operation” on February 24 last year to defend Russian speakers in eastern Ukraine and ensure “the safety and security of our country.”
“It’s becoming very clear that the Ukrainian crisis will only become a catalyst for the visceral Russophobia to come to the surface,” he said. “It has now contaminated the American and European elites” who are competing against each other to impose sanctions when in fact the sanctions are hitting the developing world hardest.
He urged member nations to vote against the resolution which will be put to a vote at the end of a special emergency session of the assembly, and said it will not help with peace negotiations.
High Representative of the European Union for Foreign Affairs, Josep Borrell, rejected the Russian envoy's assertion and stressed the war “is not a European issue … nor is it about the West versus Russia.”
“No, this illegal war concerns everyone: the North, the South, the East and the West,” he said.