The mayors of Hiroshima and Nagasaki called for ridding the world of nuclear weapons amid Russia's threats to use them in its war against Ukraine, as the first meeting of parties to a U.N. treaty prohibiting the weapons started Tuesday in Vienna.

The three-day meeting aims to build momentum toward the eventual elimination of such weapons. Japan, however, did not join even as an observer, despite high expectations among survivors of the atomic bombings of the two Japanese cities that it would.

The Japanese government has said that as no nuclear-weapon state has signed on to the pact, known officially as the Treaty on the Prohibition of Nuclear Weapons, Japan would rather promote "realistic efforts" toward a world free of nuclear weapons while trying to involve those states.