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 ultimate weapons of war started Tuesday in Vienna.

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

Photo taken in Vienna on June 21, 2022, shows the first meeting of state parties to the Treaty on the Prohibition of Nuclear Weapons. (Kyodo)

The Japanese government has said 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.

In a speech as president of the nongovernmental organization Mayors for Peace, Hiroshima Mayor Kazumi Matsui cautioned that nuclear weapons are not a solution to the war in Ukraine.

Nagasaki Mayor Tomihisa Taue, who serves as the group's vice president, said that amid the Russian nuclear threat, "now more than ever, I feel that the existence of the treaty is very important as it is the only international treaty that clearly prohibits the immediate crisis the world is now facing."

In a video message, U.N. Secretary General Antonio Guterres said, "Let's eliminate these weapons before they eliminate us."

The meeting is expected to conclude Thursday with a political statement and an action plan to chart a course for nuclear abolition and the salvation of "hibakusha" -- survivors and others who have been physically affected by nuclear radiation -- as well as the restoration of environments contaminated by nuclear tests.

The treaty is the first international pact outlawing nuclear weapons development, testing, possession and use.

The treaty, which finds the complete elimination of such weapons as the only way to guarantee nuclear weapons are never used again, took effect in January last year, with 65 parties ratifying it so far.

The United States, China, Russia and other nuclear-armed nations are not part of it, and Japan has stood aligned with the United States as it has a long-standing security alliance with Washington that offers the shelter of its nuclear umbrella.

Survivors of the 1945 U.S. atomic bombings and signatory countries were calling for Japan's attendance as an observer as the only nation to have suffered the devastating consequences of such bombings.

Germany, Norway and the Netherlands are participating with observer status even as their national defense depends on the U.S.-led nuclear umbrella as members of the North Atlantic Treaty Organization.

Sweden and Finland, which have submitted applications to join NATO in the wake of Russia's invasion of Ukraine, as well as Australia, are among the other observer participants.

The meeting is being held in the Austrian capital amid heightened security concerns in Europe in the wake of Russia's aggression against Ukraine.

Participating countries hope to take a firm, united stance on nuclear disarmament at this meeting and attend the review conference of the Nuclear Non-Proliferation Treaty, to be held in New York in August.

Japanese Prime Minister Fumio Kishida, who represents an electoral district in Hiroshima and often speaks of the vision of a world without nuclear weapons, is attending the NPT review conference, the government said Tuesday.