Ukrainian President Volodymr Zelensky has told the United Nations General Assembly that Russia is weaponising food and energy, and threatens the whole world with nuclear radiation.

"Not only is it spreading its unreliable nuclear power plant construction technologies, but it is also turning other countries' power plants into real dirty bombs.

"Look what Russia did to our Zaporizhzhia power plant," he told delegates in New York, "shelled it, occupied it and now blackmails others with radiation leaks".

In his first in-person to the assembly, President Zelensky also accused Russia of genocide by abducting hundreds of thousands of Ukrainian children from the areas of the country occupied by Russian forces.

"We are trying to get those children back home. But time, time goes by.

"Those children in Russia are taught to hate Ukraine and all ties with their families are broken.

"This is clearly a genocide," he said.

Mr Zelensky said Russia was weaponising food shortages on the global market in exchange for recognition for some, if not all, the captured territories.

Food prices globally have shot up since Russia’s invasion over Ukraine.

The two countries supply the bulk of the world’s grain and fertiliser.

In July, Russia pulled out of a deal brokered by the UN and Turkey to allow exports of Ukrainian grain from ports in the Black Sea.

President Zelensky told UN delegates that Russia would turn Ukraine into a weapon against them.

"Each decade, Russia starts a new war," he said.

"Parts of Moldova and Georgia remain occupied. Russia turned Syria into ruins and if not for Russia, the chemical weapons would have never been used there in Syria."

He said that Russia had almost swallowed Belarus and "is obviously threatening Kazakhstan and other Baltic states".

Many seats in the General Assembly hall may become empty, he said, "if Russia succeeds with its treachery and aggression".

President Zelensky said he would be bringing his peace formula to a special meeting of the UN Security Council tomorrow.

He said it offered a real chance to end aggression on the terms of the nation which was attacked.

"This is a real chance for every nation to ensure that aggression against your state, if it happens, God forbid, will not be because your land will be divided and you will be forced to submit to military or political pressure but because your territory and sovereignty will be fully restored," he said.

As he spoke, Russian Deputy Ambassador to the UN, Dmitry Polyanskiy, sat in Russia's chair but did not appear to be paying close attention, scrolling though his phone instead.

Russia claims that Ukraine and NATO started the war.

Biden condemns Russia's 'naked aggression'

US President Joe Biden addressed the super-power rivalry deadlocking the UN during his speech to the assembly.

He condemned Russia's "naked aggression" against Ukraine, warning that any member state could face a similar fate.

"If we abandon the core principles of United States to appease an aggressor, can any member state in this body feel confident that they're protected?" he asked.

"If we allow Ukraine to be carved up, is the independence of any nation secure? I respectfully suggest the answer's no."

President Biden also spoke about his commitment to managing the US' increasingly tense relationship with China, "so it does not tip into conflict".

"I've said we are for derisking, not decoupling from China," he said in a reference to recent US policies targeting Chinese imports as well as access to American technology.

"We will push back on aggression and intimidation to defend the rules of the road," he told delegates, but would work with China on areas of mutual concern, such as climate change.

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'We know our future is bound to yours' - Biden

Presidents Xi and Putin of China and Russia are conspicuous by their absence this week.

Heads of states from France and UK have also stayed away, leaving the US as the only permanent member of the UN Security Council to be represented by its leader.

Mr Biden seemed to reference his unique position saying: "I understand the duty my country has to lead this critical moment, to work with countries in every region, linking them in common cause."

Overall, in his speech, the US President struck a globalist tone.

He spoke about the US commitment to the Sustainable Development Goals - the UN's blueprint to protect the planet and improve lives - as well as the "existential threat" of climate change.

"We know our future is bound to yours," he told delegates, repeating the statement for emphasis.

He cited the recent re-entry of the US into UNESCO, the UN’s education and cultural agency, as a sign of his country’s commitment to the global body.

His comments were in stark contrast to those of his predecessor, and potential rival in next year’s presidential election, Donald Trump, who told the UN that the future did not belong to globalists, but to patriots.

"The institutions we built together at the end of the Second World War are an enduring bedrock of our progress, and the United States is committed to sustaining them," President Biden told delegates.

But, he said, the UN needed to reform, starting with the expansion of the Security Council - the UN’s most senior decision-making body.

The five permanent members of the Security Council - US, UK, China, Russia, France - have dominated the Council since 1945, wielding a veto over decisions they don’t like. Ten other non-permanent members are elected for a rotating two-year term.

Russia’s invasion of Ukraine has exposed the council’s inability to respond when a permanent member takes unilateral action.

"We need to be able to break the gridlock, too often stymies progress blocks consensus on the council," President Biden said, "we need more voices, more perspectives at the table".

Ukraine took centre stage today, but the focus on the country has drawn criticism from member states who are concerned that conflict and humanitarian crises in their own nations are not getting the same attention.

"Some leaders from non-Western nations will quietly resent the fact that Ukraine will dominate the high-level week for another year," said Richard Gowan, UN expert with the Crisis Group, a New York-based think tank.

"Zelensky will need to tread carefully at the UN, as there are a lot of non-Western countries that want to see an early diplomatic solution to the war," he told RTÉ News.

In his address, President Biden appeared to reference fatigue among members states with the war in Ukraine.

"Russia believes in the world grow weary, allowed to brutalize Ukraine without consequences.

"Russia alone has the power to end this war immediately," he told delegates.

Russia’s foreign minister Sergey Lavrov is expected to deliver his remarks to the General Assembly at the weekend.