Samantha Power said that from the United States to Europe to the Middle East "no one was ready" for the crisis.
"Because we are dealing with the largest displacement crisis since World War II, it has overwhelmed the global system," Power said.
She spoke after a tour of a way station for migrants in northern Paris, opened a month ago to shelter travelers for up to 10 days while helping them apply for asylum, then placing them in more permanent lodgings.
The center can house 400 single men - a far cry from the nearly 30,000 people taken off Paris streets over the past year. But it is the first official attempt to organize and speed up asylum bids. It opened two weeks after some 7,000 migrants, most trying to sneak into Britain, were evacuated from a camp in the English Channel port city of Calais and given temporary lodgings around France.
Power spent some 45 minutes meeting privately with four migrants, from Somalia, Pakistan and Afghanistan. One, 18-year-old Mohamed Abdi said later he had hoped to go to Britain, but is now applying for asylum in France.
Scores of weary travelers trying to get into the Paris center - which takes in only 50 per day - waited in the cold, many saying they had slept nearby on a sidewalk to ensure a place in line, or in a park. A center for women and children is to open in early 2017 just south of Paris.
Power praised the center, which includes a health clinic, as an example of trying to bring order to chaotic lives. She said she believes the mass surge of the displaced onto an unprepared world stoked people's fear and fed "the sense that the movement was out of control."
Today, nearly two years after people uprooted by war, despots or fear for their lives began pouring into Europe, other forces are at work. Nationalists in Europe and the United States are making their voices heard.
In France, Marine Le Pen, leader of the anti-immigrant National Front party, is a favorite in spring presidential elections, and has said that "perhaps" she could benefit from the victory of U.S. President-elect Donald Trump and tagged Britain's Brexit vote "the most important historic event since the fall of the Berlin Wall."
Power said she cannot predict whether the Trump administration will scale back the Obama administration's target of taking in 110,000 refugees for fiscal year 2017. Congress must approve funding.
"We have put in place the most stringent and rigorous vetting procedures for the refugee population," she said. During the presidential campaign, Trump called for "extreme vetting" of migrants coming to the U.S.
Power said nations neighboring Syria, with fragile infrastructures, have shown generosity for Syrians fleeing the war ravaging their country, and noted that one in every five Lebanese residents today is a Syrian refugee.
The U.N. ambassador issued a word of caution as some Western nations turn inward, saying that allowing the crisis to fester can be dangerous.
"We should avoid the mistake of latching onto the idea that if these populations are (consigned) to one part of the world that there's no security threat in that and no risk in that.
"If countries do not have the wherewithal to absorb this population ... this is going to be a security challenge that the United States, France and the world will be dealing with for generations to come."
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.