US President Joe Biden took to the world stage Wednesday to single out Russian President Vladimir Putin for brazen actions that he said are in direct violation of the UN's founding principles.

Addressing the 77th UN General Assembly, the US president said Putin has "shamelessly violated the core tenants of the United Nations Charter" when he authorized Russia's forces to invade Ukraine in February -- a decision that is a violation of the "clear prohibition against countries taking the territory of their neighbor by force."

The war, Biden said, is aimed at "extinguishing Ukraine's right to exist as a state plain and simple, and Ukraine's right to exist as a people."

"Just today, President Putin has made overt nuclear threats against Europe in a reckless disregard for the responsibilities of the non-proliferation regime," he said as world leaders convened at the UN's New York headquarters.

"Now, Russia's calling up more soldiers to join the fight. The Kremlin is organizing sham referenda, trying to annex parts of Ukraine, an extremely significant violation of the UN Charter," he added.

Putin announced earlier Wednesday a mass mobilization in Russia of 300,000 reservists to support his flagging war effort in Ukraine. In doing so he offered a dire warning ahead of planned referenda in parts of Ukraine denounced by the West as a "sham" effort to annex additional Ukrainian territory.

"When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. This is not a bluff," said Putin.

The move comes one day after the announcement of referenda in the Ukrainian regions of Donetsk, Luhansk, Kherson and parts of Zaporizhzhia. Putin and his regional proxies are seeking to use the polls to claim the territories as part of Russia.  

Support for expansion of UN Security Council

Turning to reform of the UN Security Council amid ongoing deadlocks on key issues, Biden voiced support for expanding the body's membership, saying permanent seats should be granted to nations in Africa, Latin America and the Caribbean.

"The United States is committed to this vital work," he said, emphasizing that permanent members should refrain from using their veto powers "except in rare, extraordinary situations to ensure the Council remains credible and effective."

The president announced an additional $2.9 billion in US humanitarian assistance to support the global food crisis exacerbated by Russia's war on Ukraine, saying the Kremlin "in the meantime, is pumping out lies, trying to pin the blame for the food crisis on sanctions imposed by many in the world for the aggression against Ukraine."

"Let me be perfectly clear about something. Our sanctions explicitly allow Russia the ability to export food and fertilizer, no limitation," he said. "Russia's war is worsening food insecurity, and only Russia can end it."  

China in midst of 'unprecedented' nuclear buildup

Even as he singled out Putin for threatening Europe with doomsday weapons, Biden said China is in the midst of a rapid advancement of its nuclear program.

"China is conducting an unprecedented and concerning nuclear buildup without any transparency," he said, maintaining that the US stands ready to further strengthen its nuclear non-proliferation commitments "no matter what else is happening in the world."

"A nuclear war cannot be one and must never be fought," he said.

The president continued to voice support for a negotiated two-state solution to the Israel-Palestine conflict, saying it remains "the best way to ensure Israel's security and prosperity for the future and give the Palestinians the state to which they are entitled."​​​​​​​