UNITED NATIONS -- Insisting that international cooperation is critical, the United Nations chief delivered a dire warning to leaders from across the world Tuesday, declaring that the planet is becoming unhinged with mounting global challenges and geopolitical tensions -- and warning that "we seem incapable of coming together to respond."

Addressing presidents and prime ministers, monarchs and ministers at the opening of the UN General Assembly's high-level meeting, Secretary-General Antonio Guterres ticked off a list of "existential threats" the world is facing, from climate change to disruptive technologies like artificial intelligence.

"Our world is becoming unhinged. Geopolitical tensions are rising. Global challenges are mounting. And we seem incapable of coming together to respond," Guterres told the people who run the world's nations. He said that the United Nations -- and the ways that countries cooperate -- must evolve to meet the era.

"The world has changed. Our institutions have not," Guterres said before the opening of the UN General Assembly's General Debate. "We cannot effectively address problems as they are if institutions don't reflect the world as it is. Instead of solving problems, they risk becoming part of the problem."

All this is taking place. Guterres said, as the world is making a "chaotic transition" and rapidly moving from a brief period of "unipolarity" -- domination by a single power, the United States -- toward a multipolar world with many power centers. That is, he said, positive in many ways.

ADVOCATING AN EFFECTIVE 'MULTIPOLAR' WORLD

Guterres said a multipolar world needs strong, effective multilateral institutions where all countries work together to solve the world's challenges. But the current institutions formed on the ashes of World War II, including the United Nations and its powerful Security Council and key global financial institutions, have not changed enough.

If these institutions are not reformed to reflect the world today, Guterres said the alternative is not maintaining the status quo; it is "further fragmentation." He added: "It's reform or rupture."

Guterres warned that divides are deepening, among economic and military powers, between countries in the developed North and developing South, and between the global West and East.

"We are inching ever closed to a Great Fracture in economic and financial systems and trade relations," he said, "one that threatens a single, open internet. (One) with diverging strategies on technology and artificial intelligence, and potentially clashing security frameworks."

He said the world needs action now -- not merely more words -- and compromise to tackle the world's challenges and adopt needed reforms.

LOTS OF LEADERS, BUT KEY ONES MISSING

This year's week-long high-level UN gathering, the first full-on meeting of world leaders since the COVID-19 pandemic disrupted travel, has 145 leaders scheduled to speak. It's a large number that reflects the multitude of crises and conflicts.

But for the first time in years, U.S. President Joe Biden, who spoke soon after Guterres, is be the only leader from the five powerful veto-wielding nations on the UN Security Council to address the 193-member assembly.

China's Xi Jinping, Russia's Vladimir Putin, France's Emmanuel Macron and Britain's Rishi Sunak are all skipping the UN this year. That should put the spotlight on Ukraine's President Volodymyr Zelenskyy, who will be making his first appearance at the assembly's podium later Tuesday, and on Biden, who will be watched especially for his views on China, Russia and Ukraine.

Guterres sharply criticized Russia's invasion of Ukraine, telling world leaders it is "Exhibit A" of countries breaking their pledge to uphold the UN Charter's pledge for peace -- and the mandate to guarantee the sovereignty and territorial integrity of all member nations.

Later, Biden echoed the sentiment. "I ask you this: If we abandon the core principles of the United States to appease an aggressor, can any member state in this body feel confident that they are protected?" Biden said in his address. "If we allow Ukraine to be carved up, is the independence of any nation secure?

He continued: "I'd respectfully suggest the answer is no."

This year's week-long session, the first full-on meeting of world leaders since the COVID-19 pandemic disrupted travel, has 145 leaders scheduled to speak. It's a large number that reflects the multitude of crises and conflicts.

The absence of leaders from the four Security Council powers has caused grumbling from developing countries who want major global players to listen to their demands -- including for money to start closing the growing gap between the world's haves and have-nots.

The G77, the major UN group of developing countries that now has 134 members including China, lobbied hard to make this year's global gathering focus on the 17 UN goals adopted by world leaders in 2015. Those are badly lagging at the halfway point to their 2030 due date.

At a two-day summit to kick-start action to achieve the goals, Guterres pointed to grim findings in a UN report in July. He said 15 per cent of some 140 specific targets to achieve the 17 goals are on track. Many are going in the wrong direction, and not a single one is expected to be achieved in the next seven years.

HE TOLD OF A 'SAD SNAPSHOT' OF THE WORLD

Guterres opened his state-of-the-world address using the massive rainfall and dam collapses in the Libyan city of Derna as "a sad snapshot of the state of our world." Thousands of people lost their lives -- victims of years of conflict, climate chaos, leaders near and far who failed to restore peace, and all that "indifference."

He said the world needs to deal with the worsening climate emergency, escalating conflicts, "dramatic technological disruptions" and a global cost-of-living crisis that is increasing hunger and poverty.

At a two-day summit to kick-start action to achieve the goals, Guterres pointed to grim findings in a UN report in July. He said 15 per cent of some 140 specific targets to achieve the United Nations' 17 "Sustainable Development Goals" are on track. Many are going in the wrong direction, and not a single one is expected to be achieved in the next seven years.

The wide-ranging goals include end extreme poverty and hunger, ensure every child gets a quality secondary education, achieve gender equality and make significant inroads in tackling climate change -- all by 2030.

At the current rate, the report said, 575 million people will still be living in extreme poverty and 84 million children won't even be going to elementary school in 2030 -- and it will take 286 years to reach equality between men and women.

Leaders from the 193 UN member nations adopted by consensus a political declaration that recognizes the goals are "in peril." But it reaffirms more than a dozen times, in different ways, leaders' commitment to achieve the goals, reiterating their individual importance.

The declaration is short on specifics, but Guterres said he was "deeply encouraged" by its commitment to improving developing countries' access to "the fuel required for SDG progress: finance." He pointed to its support for a stimulus of at least US$500 billion a year to boost the goals, aimed at offsetting challenging market conditions faced by developing countries.