Speaking before the UN Human Rights Council in Geneva, Michelle Bachelet warned that Russia's full-scale invasion of Ukraine, launched a week ago, "is generating massive impact on the human rights of millions of people across Ukraine."

"Elevated threat levels for nuclear weapons underline the gravity of the risks to all of humanity," she added.

Her comments, during an urgent council debate on the Ukraine conflict, came after Russian President Vladimir Putin on Sunday ordered Russia's nuclear forces be put on high alert.

Earlier on Thursday, Russia's Foreign Minister Sergei Lavrov accused Western politicians of fixating on nuclear war.

"It is clear that World War Three can only be nuclear," Lavrov said in an online interview with Russian and foreign media.

"I would like to point out that it's in the heads of Western politicians that the idea of a nuclear war is spinning constantly, and not in the heads of Russians," he said.

Moscow has the world's largest arsenal of nuclear weapons and a huge cache of ballistic missiles which form the backbone of the country's deterrence forces.

Bachelet's speech came as UN figures showed the devastating week-old war had already forced more than one million people to flee Ukraine into neighbouring nations, with countless others displaced inside the war-ravaged country.

Bachelet said her office had recorded 227 civilian deaths, including at least 15 children, but stressed that the real numbers were likely far higher.