UNITED NATIONS -Countries have started signing on to the first treaty to ban nuclear weapons that is backed by over 100 nations. But the nuclear powers want no part of it.

Brazilian President Michel Temer was first to sign at a ceremony Wednesday at the United Nations. Its treaty office said 51 countries were expected to sign on the opening day.

U.N. Secretary-General Antonio Guterres calls the pact "an important step towards the universally held goal of a world free of nuclear weapons."

More than 120 countries approved the treaty in July over opposition from nuclear-armed countries and their allies. They boycotted negotiations.

Supporters of the pact say it's time to push harder toward eliminating atomic weapons than nations have done through the nearly 50-year-old Nuclear Non-Proliferation Treaty.

Nuclear powers say a ban won't work.