UN Secretary-General Antonio Guterres on Sunday issued a year-end "red alert" to a world he said faces the perils of inequality, nationalism, xenophobia and possibly even nuclear war unless it works to resolve its differences.

"When I took office one year ago, I appealed for 2017 to be a year for peace," he said in his New Year's message. "Unfortunately, in fundamental ways, the world has gone in reverse.

"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.

He suggested that the international community "can settle conflicts, overcome hatred and defend shared values, but we can only do that together."

Guterres urged world leaders: "Narrow the gaps. Bridge the divides. Rebuild trust by bringing people together around common goals."