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World leaders looked to Nelson Mandela's legacy of championing peace, human rights and global cooperation but acknowledged the world is far from achieving his ideals as the U.N. General Assembly's annual top-level meeting began Monday.

At a peace summit honoring the late South African leader, nations from around the world adopted a declaration recommitting to goals of building a peaceful, inclusive and fair world and "to revive the values for which Nelson Mandela stood" by emphasizing human dignity. At the same time, they worried that the idea of taking multinational action to solve major problems is facing increasing doubt.

"As leaders of this time, you have moral imperative and the ability to bring the death and destructions we witness on a daily basis to an end," Mandela's widow, Graca Machel, told the heads of state and U.N. officials. She implored them to take on "ego-driven" decision-makers, political dogma, greed and the arms industry.

"Humankind will hold you accountable should you allow suffering to continue on your watch," she said.

The appeal for peace and collaboration comes as the U.N.'s founding concepts of shared values and responsibility are being tested, from the "America First" agenda of U.S. President Donald Trump to the U.K.'s impending divorce from the European Union and more.

Trump, arriving at the U.N. Monday for a meeting on drugs, highlighted progress on a key issue for global peace: the nuclear threat from North Korea.

He announced he will likely hold a second summit "quite soon" with the North's leader, Kim Jong Un. Trump said it's "a much different time" from last year's General Assembly meeting, when Trump derided Kim as "Rocket Man."

Still, some of the tensions underlying the global confab weren't hard to detect.

"Great statesmen tend to build bridges instead of walls," Iranian President Hassan Rouhani said at the peace summit. Trump — who pulled the U.S. out of Iran's nuclear deal with world powers — campaigned on a promise to build a wall along the U.S.-Mexico border.

This year marks the 100th anniversary of Mandela's birth, and the U.N. is declaring 2019-2028 the "Nelson Mandela Decade of Peace." A $1.8 million statue of a smiling Mandela with outstretched arms was unveiled at U.N. headquarters Monday.

Imprisoned in South Africa for 27 years, Mandela became the international face of the struggle to end the country's apartheid system of white minority rule over the majority black population.

Four years after he walked out of jail, he became the country's first black president in its first multi-racial elections. Over the ensuing decades, he became a Nobel peace laureate and global statesman.

In a speech at the U.N. in 1994, he said its challenge was "to answer the question - given the interdependence of the nations of the world — what is it that we can and must do to ensure that democracy, peace and prosperity prevail everywhere!"

His question is all the more pressing now, U.N. leaders said.

"With human rights under growing pressure around the world, we would be well served to reflect on the example of this outstanding man," U.N. Secretary-General Antonio Guterres said.

U.N. General Assembly President Maria Fernanda Espinosa Garces said Mandela "represents a light of hope for a world still torn apart by conflicts and suffering" — but one where there are concerns about the international community's ability to work together to resolve such major problems as poverty, hunger, war and global warming.

"Drifting away from multilateralism means jeopardizing the future of our species and our planet," she said. "The world needs a social contract based on shared responsibility, and the only forum that we have to achieve this global compact is the United Nations."