Japan marked the 72nd anniversary of the U.S. nuclear attack on its western city of Hiroshima Sunday, as Prime Minister Shinzo Abe and survivors attended an annual ceremony.

Thousands of participants observed a minute of silence at 8:15 a.m. in memory of the victims, the exact moment a U.S. B-29 bomber dropped an atomic bomb on the city on Aug. 6, 1945.

The blast killed tens of thousands of residents instantly, and by the end of the year, some 140,000 in total had died.

This year's ceremony followed the adoption of the first treaty to comprehensively prohibit nuclear weapons, with 122 United Nations members adding their signatures.

Hiroshima Mayor Kazumi Matsui praised the treaty's adoption, saying the countries "demonstrated their unequivocal determination to achieve abolition."

"This hell is not a thing of the past. As long as nuclear weapons exist and policymakers threaten their use, their horror could leap into our present at any moment. You could find yourself suffering their cruelty," the mayor stressed, adding, "Given this development, the governments of all countries must now strive to advance further toward a nuclear-weapon-free world."

U.N. Secretary General Antonio Guterres penned a message for the ceremony.

"This measure is the result of a global campaign focused on the unacceptability of the use of nuclear weapons under any circumstances," the UN chief said, referring to the treaty.

"Yet our dream of a world free of nuclear weapons remains far from reality. The states possessing nuclear weapons have a special responsibility to undertake concrete and irreversible steps in nuclear disarmament," he added.

Japan refused to join the treaty, along with the nuclear weapon states and several U.S. allies.

Prime Minister Shinzō Abe avoided touching on the treaty directly in his speech at Sunday's event.

"For us to truly realize a 'world without nuclear weapons,' the participation of both nuclear weapon states and non-nuclear weapon states is necessary," Abe said.

In 1945, the United States dropped a second atomic bomb on Japan, on Nagasaki on the island of Kyushu, killing another 70,000 three days after the Hiroshima bombing. Japan surrendered on Aug. 15, ending World War II.

Barack Obama traveled to Hiroshima in May 2016, becoming the first incumbent U.S. president to visit the site of the deadly bombing. He issued sympathy for the victims, but did not offer an apology for the attack.

Many Japanese and others in North Asia seem resigned to North Korea's apparent newfound capacity to launch missiles capable of reaching, not only Japan, but much of the continental United States as well.