UNITED NATIONS — Saying the time was not right to outlaw nuclear arms, the United States led a group of dozens of United Nations members on Monday that boycotted talks at the global organization for a treaty that would ban the weapons.

“There is nothing I want more for my family than a world with no nuclear weapons,” Ambassador Nikki R. Haley of the United States told reporters outside the General Assembly as the talks began. “But we have to be realistic. Is there anyone who thinks that North Korea would ban nuclear weapons?”

Ms. Haley and other ambassadors standing with her, including envoys from Albania, Britain, France and South Korea, declined to take questions.

The talks, supported by more than 120 countries, were first announced in October and are led by Austria, Brazil, Ireland, Mexico, South Africa and Sweden. Disarmament groups strongly support the effort.