UN Secretary-General Antonio Guterres on Monday called for an immediate end to the military operations in Ukraine.

"The fighting in Ukraine must stop. It is raging across the country, from air, land and sea. It must stop now," he told an emergency special session of the UN General Assembly on Ukraine.

"Enough is enough. Soldiers need to move back to their barracks. Leaders need to move to peace. Civilians must be protected. International humanitarian and human rights law must be upheld. The sovereignty, independence and territorial integrity of Ukraine, within its internationally recognized borders, must be respected," said Guterres.

"We are facing a tragedy for Ukraine, but also a major regional crisis with potentially disastrous implications for us all," he warned. "Yesterday, Russian nuclear forces were put on high alert. This is a chilling development. The mere idea of a nuclear conflict is simply inconceivable. Nothing can justify the use of nuclear weapons."

The only true solution is peace, said Guterres. "The guns are talking now, but the path of dialogue must always remain open. It is never too late to engage in good-faith negotiations and to address all issues peacefully."

He expressed the hope that the direct talks now taking place between Ukrainian and Russian delegations will produce not only an immediate halt to the fighting, but also a path toward a diplomatic solution.

"I welcome and encourage all peaceful efforts to end the bloodshed and halt this conflict. I thank the countries who have offered to host and facilitate negotiations. The United Nations is ready to support such efforts," he said.

"War is not the answer. It is death, human suffering, senseless destruction and an immense distraction from the real challenges facing humanity," he said.