US President Joe Biden has declared at the United Nations that Russia has "shamelessly violated the core tenets" of the international body with its war in Ukraine as he summoned nations around the globe to stand firm in backing the Ukrainian resistance.
Delivering a forceful condemnation on Wednesday of Russia's seven-month invasion, Biden said reports of Russian abuses against civilians and its efforts to erase Ukraine and its culture "should make your blood run cold".
He referenced President Vladimir Putin's announcement on Wednesday that he had ordered a partial mobilisation of reservists, a deeply unpopular step that sparked protests in Russia.
And Putin's new nuclear threats against Europe showed "reckless disregard" for Russia's responsibilities as a signer of the Treaty on the Non-Proliferation of Nuclear Weapons, Biden said.
He also criticised Russia for scheduling "sham referenda" this week in territory it has forcibly seized in Ukraine.
"A permanent member of the UN Security Council invaded its neighbour, attempted to erase a sovereign state from the map. Russia has shamelessly violated the core tenets of the UN charter," he told his UN audience.
Biden's speech was part of an effort to maintain Russia's isolation on the global stage as the costs of war mount, worries about energy this winter loom and Congress is likely to look more sceptically on spending more on military defence.
The president called on all nations, whether democracies or autocracies, to speak out against Russia's "brutal, needless war" and to bolster's Ukraine effort to defend itself.
"We will stand in solidarity against Russia's aggression, period," Biden said.
Biden, during his time at the UN General Assembly, met with Secretary General Antonio Guterres and held his first meeting with British Prime Minister Liz Truss, during which they discussed Russia's war, energy security and China. He also met with French President Emmanuel Macron and South Korean President Yoon Suk Yeol.
Biden also pressed nations to meet an $US18 billion ($A27 billion) target to replenish the Global Fund to Fight AIDS, tuberculosis and malaria, reiterating the US commitment of $US6 billion ($A9 billion) to that goal.
"Now is the moment to accelerate our efforts to reduce health inequities, and to address barriers to access including gender and human rights barriers to build a more inclusive healthcare systems, to leave no one behind, to end AIDS, tuberculosis and malaria for good," Biden said, as he was joined by leaders from Japan, Germany, France, Canada and the EU for the announcement.
But the heart of the president's visit to the UN this year was his full-throated censure of Russia as its war nears the seven-month mark.
The address came as Russian-controlled regions of eastern and southern Ukraine have announced plans to hold Kremlin-backed referendums on becoming part of Russia and as Moscow was losing ground in the invasion.
Biden was confronting no shortage of difficult issues as leaders gathered this year.
In addition to the Russian war in Ukraine, European fears that a recession could be just around the corner are heightened. Administration concerns grow by the day that time is running short to revive the Iran nuclear deal and over China's sabre-rattling on Taiwan.
Biden's visit to the UN also comes as his administration's efforts to revive the 2015 Iran nuclear deal appear stalled.
The deal brokered by the Obama administration -- and scrapped by Trump in 2018 -- provided billions of dollars in sanctions relief in exchange for Iran's agreement to dismantle much of its nuclear program and open its facilities to extensive international inspection.
"While the United States is prepared for a mutual return to the Joint Comprehensive Plan of Action, if Iran steps up to its obligations, the United States is clear: We will not allow Iran to acquire nuclear weapons," Biden said.
This year's UN gathering is back to being a full-scale, in-person event after two years of curtailed activity due to the COVID-19 pandemic.