The head of the United Nations warned on Tuesday of a bleak future caused by rising prices, a warming planet and deadly conflicts as world leaders sought ways forward on Ukraine and Iran.

The UN General Assembly, the annual gathering of world leaders that clogs Midtown Manhattan, returned in person after two years of pandemic restrictions with only one leader allowed to appear virtually -- Ukrainian President Volodymyr Zelensky.

UN Secretary-General Antonio Guterres opened the summit with an image of a ship carrying grain out of Ukraine, a symbol of successful diplomacy, but he warned of a dire state of the planet.

"A winter of global discontent is on the horizon," Guterres said.

"Trust is crumbling, inequalities are exploding, our planet is burning. People are hurting -- with the most vulnerable suffering the most."

With global temperatures rising and a chunk of Pakistan the size of the United Kingdom recently under water, Guterres lashed out at fossil fuel companies and the "suicidal war against nature."

"Let's tell it like it is -- Our world is addicted to fossil fuels. It's time for an intervention. We need to hold fossil fuel companies and their enablers to account," Guterres said.

He called on all developed economies to tax profits from fossil fuels and dedicate the funds both to compensate for damage from climate change and to help people struggling with high prices.

"Polluters must pay," Guterres said.

The summit still saw disruption due to the death of Queen Elizabeth II, with President Joe Biden of the United States, by tradition the second speaker on the opening day, instead due to speak on Wednesday.

Just as leaders huddled at the United Nations about the war in Ukraine, Russian-backed forces announced they were going ahead with a move the West had long warned against -- referendums on annexation by Moscow.

German Chancellor Olaf Scholz called the votes, to be conducted in the coming days in Russian-occupied Ukrainian territory, a "sham" that were part of "imperialist aggression" by Moscow.

The war in Ukraine, a major grain producer, has sent global food prices spiraling, hitting developing nations especially hard.

Senegalese President Macky Sall, the current chair of the African Union, urged a "negotiated solution" in Ukraine to "avoid the catastrophic risk of a potentially global conflict."

Turkish President Recep Tayyip Erdogan, who has fashioned himself as a mediator and played a key role in arranging the grain shipments, called for an end to the war that recognizes Ukraine's sovereignty and territorial integrity.

"Together, we need to find a reasonably practical diplomatic solution that will give both sides a dignified way out of the crisis," Erdogan told the General Assembly.

Western leaders led by the United States have made clear they do not want the summit to focus exclusively on the Ukraine war itself, mindful of resentment in the developing world to the billions of dollars sent in weapons.

"The brutality of Russia's war of aggression and its threat to the peace order in Europe have not blinded us to the fact that its dramatic effects are also clearly being felt in many other regions of the world," German Foreign Minister Annalena Baerbock said.

US Secretary of State Antony Blinken will head a meeting on food security, and Guterres on Monday launched a bid to step up funding for education, badly affected by the pandemic. (AFP)