US President Joe Biden delivered a firm rebuke of Russia's "brutal, needless war" in Ukraine during an address to the United Nations General Assembly in New York on Wednesday.

He said Moscow "shamelessly violated" the UN charter with the invasion.

"If nations can persue their imperial ambitions without consequences then we put at risk everything this institution stands for," he said.

Biden's address to the UN comes only hours after Russian President Vladimir Putin ordered reservists to report for duty to fight the war in Ukraine.

He also said Putin's nuclear threats against Europe showed "reckless disregard'' for his nation's responsibilities as a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons.

"We will stand in solidarity against Russia's aggression. Period,'' he vowed. 

The US leader urged world leaders to show "clear, firm and unwavering" resolve against the war.

"This is war is about extinguishing as Ukraine's right as a state...and as a people," he said.

Biden announces new global food security assistance 

He also announced $2.9 billion in global food security aid to address shortages caused by the war in Ukraine and the effects of climate change.

The White House said $2 billion in direct humanitarian assistance would be distributed through the United States Agency for international Development.

The balance of the money will go to global development projects meant to boost the efficiency and resilience of the global food supply.

Biden on US-China tensions: 'We do not seek conflict' 

Biden also addressed ongoing tensions between the US and China following House Speaker Nancy Pelosi's visit to Taiwan. 

"Let me be direct about the competition between the United States and China," Biden said. "As we manage shifting geopolitical trends, the United States will conduct itself as a reasonable leader. We do not seek conflict, we do not seek a Cold War."

The president said the US would not force countries to "choose" sides between between Washington and Beijing.     

In addition, Biden said the US supports the expansion of the UN Security Council to better represent areas including Africa and Latin America.

"This includes permanent seats for those nations we've long supported -- permanent seats for countries in Africa, Latin America, the Caribbean. The United States is committed to this vital work," he added.

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

lo/wd (AFP, AP, Reuters, dpa)