Negotiations on a treaty outlawing nuclear weapons began Monday at the U.N. headquarters without Japan, as the world's only atomic-bombed country said it would abstain from the talks alongside the five major nuclear powers.

The decision triggered criticism and disappointment from survivors of the 1945 U.S. atomic bombings of Hiroshima and Nagasaki, who saw the first-ever U.N. talks on the treaty as a step toward pursuing a world free of nuclear weapons.

"Regrettably, given the present circumstances, we must say that it would be difficult for Japan to participate ... in a constructive manner and in good faith," disarmament ambassador Nobushige Takamizawa said during the opening segment of the conference. "We will continue to pursue realistic and effective disarmament measures and will work to create a security environment conducive to the elimination of nuclear weapons."