He said this during a high-level UN Security Council meeting on Thursday, according to Ukrinform's correspondent in New York.
"Russia's war on Ukraine shows no sign of letting up. The past seven months have seen unspeakable suffering and devastation," he said.
According to Guterres, the idea of a nuclear conflict, "once unthinkable," has become a subject of debate. "This in itself is totally unacceptable. All nuclear-armed states should recommit to the non-use and total elimination of nuclear weapons," he added.
Guterres said he was "deeply concerned" by reports of plans to organize so-called referenda in the temporarily occupied territories of Ukraine. 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, he said.
He noted that thousands of Ukrainian civilians, including hundreds of children, had been killed or injured, mostly by Russian bombardment of urban areas. "Every day, an average of five children are killed or injured. Almost every child in Ukraine has been scarred by the nightmare of war - from violence to family separation," he said.
According to Guterres, some 14 million people have been forced to flee the war, the majority of them women and children.
"The situation will only get worse with winter approaching and gas and electricity supplies dwindling," he said.