The US Ambassador to the United Nations Nikki Haley said Britain, France and the US are among the nearly 40 nations that have not taken part in talks on a nuclear weapons ban that started on Monday.
The negotiations taking place at the UN in New York seek to ban nuclear weapons in international law.
Haley told reporters the countries skipping the negotiations are instead committed to the Non-Proliferation Treaty, which entered into force in 1970 and is aimed at preventing the spread of nuclear weapons and weapons technology.
"There is nothing I want more for my family than a world with no nuclear weapons. But we have to be realistic. Is there anyone that believes that North Korea would agree to a ban on nuclear weapons?" Haley said.
"You are going to see almost 40 countries that are not in the General Assembly today," Haley said.
"In this day and time, we can't say honestly that we can protect our people by allowing the bad actors to have them and those of us that are good, trying to keep peace and safety, not to have them."
Britain's UN Ambassador Matthew Rycroft said: "The UK is not attending the negotiations on a treaty to prohibit nuclear weapons because we do not believe that those negotiations will lead to effective progress on global nuclear disarmament."
Deputy French UN Ambassador Alexis Lamek said the security conditions were not right for a nuclear weapons ban treaty.
"In the current perilous context, considering, in particular, the proliferation of weapons of mass destruction and their means of delivery, our countries continue to rely on nuclear deterrence for security and stability," Lamek said.
Activists who support a ban on nuclear weapons were hoping that if more countries could come together on the issue it would act as a deterrence for those who have nuclear ambitions.