New York, New York - President Joe Biden delivered a scathing broadside against Russia at the United Nations on Wednesday, saying Moscow had launched an invasion aimed at "extinguishing" Ukraine and that "horrifying evidence" of war crimes existed.

"This war is about extinguishing Ukraine's right to exist as a state, plain and simple. And Ukraine's right to exist as a people," he said at the UN headquarters in New York on Wednesday.

"Whoever you are, wherever you live, whatever you believe – that should make your blood run cold."

Russia must be held accountable for any war crimes committed in Ukraine, Biden continued, saying Washington was working on the issue with its international partners.

Biden spent a significant amount of time talking about Russia's "brutal, needless war" in Ukraine, which has seen Kiev notch a number of recent battlefield victories as it aims to recapture territory seized by Russia in the nearly seven-month-old conflict.

The US leader spoke just hours after Russian President Vladimir Putin ordered a partial mobilization of reservists. The Kremlin chief also gave his support for Russian-controlled areas of Ukraine to become part of the Russian Federation.

Biden told the leaders gathered at the 77th General Debate of the UN General Assembly (UNGA) that Moscow had "shamelessly violated" the UN charter, which outlines the organization's fundamental principles, and that the world must be "firm and unwavering" in denouncing the war.

He also touched on the topic of nuclear threats, following Putin's promise that Russia would use any means necessary to defend itself. "This is not a bluff," the Russian leader said earlier on Wednesday.

"A nuclear war cannot be won and must never be fought," Biden said.

In a break with tradition, Biden's appearance came on the second day of the high-level annual diplomatic event instead of the first, with the delay due to Biden's attendance of Queen Elizabeth II's state funeral in London on Monday.

Beyond Russia, Biden touched on issues including relations with China, food insecurity, and the climate crisis.

"We don't have much time left," Biden said. "We all know we're already living in a climate crisis."

"Pakistan is still underwater and needs help. Meanwhile, the Horn of Africa faces unprecedented drought," Biden said. "Families are facing impossible choices, choosing which child to feed and wondering whether they'll survive. This is the human cost of climate change. And it's growing, not lessening."

UN Secretary General António Guterres and Brazilian President Jair Bolsonaro were among those who gave speeches to the General Assembly on Tuesday. Over 140 heads of state and government are expected to have attended the week-long event by the time it ends next week.

Russian President Vladimir Putin is not attending, however, but has sent Foreign Minister Sergei Lavrov to represent Moscow instead.