United Nations Secretary General Antonio Guterres warned on Sunday of threats facing the world in 218, while Chinese leader Xi Jinping pledged to “strongly” back the organization.
Guterres said he took office a year ago with an appeal that 2017 would be a year of peace.
"Unfortunately — in fundamental ways, the world has gone in reverse," he said. "On New Year's Day 2018, I am not issuing an appeal. I am issuing an alert — a red alert for our world."
"Conflicts have deepened and new dangers have emerged. Global anxieties about nuclear weapons are the highest since the Cold War," an allusion to the crisis over North Korea's nuclear tests and missile firings.
"Climate change is moving faster than we are. Inequalities are growing. We see horrific violations of human rights," said the former Portuguese prime minister, expressing concern as well over a rise in nationalism and xenophobia.
Nationalism and xenophobia are on the rise, inequality is growing, climate change is accelerating and the world is seeing horrific violations of human rights, he said. Global anxieties about nuclear weapons were the highest since the Cold War.
He suggested that the international community "can settle conflicts, overcome hatred and defend shared values, but we can only do that together."
Guterres was the UN High Commissioner for Refugees from 2005 to 2015. During his first year in office, he has contended with humanitarian crises in Myanmar, Yemen, Syria and elsewhere.
Guterres urged world leaders: "Narrow the gaps. Bridge the divides. Rebuild trust by bringing people together around common goals."
He concluded: "Unity is the path."
For his part, Xi said on Sunday that China will play its part in defending the international order and combating climate change while working to raise living standards for its own people.
In a New Year speech, he stated that the nation was committed to economic reforms in 2018, the 40th anniversary of the transformation led by Deng Xiaoping, "as reform and opening-up is the path we must take to make progress in contemporary China and to realize the Chinese dream".
In a speech quoted by the state Xinhua news agency, Xi said that by 2020 all rural residents living below the current poverty line should have been lifted out of poverty.
It would be the first time in thousands of years of Chinese history that extreme poverty had been eliminated, he said.
"It is our solemn promise," Xi said.
The president also acknowledged shortfalls in the government's work.
"That is why we should strengthen our sense of responsibility and do a good job of ensuring the people's well-being," Xinhua quoted him as saying.
"The well-being of our people is the Party and the government's greatest political achievement."
On international affairs, Xi said China "will resolutely uphold the authority of the United Nations", actively fulfill the nation's international obligations and remain firmly committed to its pledges to tackle climate change.