The US, Britain and France were among the countries that skipped a UN meeting to discuss a new treaty.
More than 120 others endorsed a plan for a legally binding nuclear ban.
But US envoy Nikki Haley said national security required nuclear arms because of "bad actors" who could not be trusted.
"There is nothing I want more for my family than a world with no nuclear weapons. But we have to be realistic," she told reporters.
"Is there anyone that believes that North Korea would agree to a ban on nuclear weapons?"
