The mayor of Nagasaki on Thursday urged nuclear states and countries dependent on them to shift from security policies reliant on nuclear deterrence, as the city marked the 73rd anniversary of the U.S. atomic bombing.

Mayor Tomihisa Taue, in his peace declaration, pressed Japan to fulfill its "moral obligation" to lead efforts toward a nuclear-free world, and also urged the government to support an international nuclear ban treaty adopted last year.

"I hereby ask that the Government of Japan, the only country to have suffered from the wartime use of nuclear weapons, support the Treaty on the Prohibition of Nuclear Weapons and fulfill its moral obligation to lead the world towards denuclearization," Taue said.

The peace declaration called on global leaders not to forget "the resolve" of the first U.N. General Assembly resolution to aim for the elimination of nuclear arsenals.

(Antonio Guterres)

U.N. Secretary General Antonio Guterres became the first sitting chief of the international body to attend the annual event as atomic bomb survivors, their family members and delegates from nuclear powers and non-nuclear states looked on.

A moment of silence was observed at 11:02 a.m., when the plutonium-core atomic bomb "Fat Man" dropped by a U.S. B-29 aircraft exploded over Nagasaki on Aug. 9, 1945, in World War II. The atomic bomb, which followed one dropped on Hiroshima on Aug. 6, killed an estimated 74,000 in Nagasaki by the end of that year.

"To the great concern of those in the atomic-bombed cities, a shift towards openly asserting that nuclear weapons are necessary and that their use could lead to increased military might is once again on the rise," Taue said.

The ceremony comes amid survivors' expectations for progress on denuclearization after the unprecedented U.S.-North Korea summit talks earlier this year.

The Nagasaki mayor said the atomic-bombed cities have great expectations for "irreversible denuclearization," urging the Japanese government to seize the opportunity to create a nuclear-free zone in Northeast Asia.

Representatives from 71 countries gathered at the Peace Park where atomic bomb survivors and visitors prayed for lasting peace.

Guterres, in his speech, called on all countries to commit to nuclear disarmament and make "visible" progress with urgency.

"Disarmament processes have slowed and even come to a halt," Guterres said as the adoption last year of the Treaty on the Prohibition of Nuclear Weapons demonstrated the frustration felt by many states.

The U.N. chief expressed solidarity with hibakusha, or atomic bomb survivors, who have become leaders of peace and disarmament. "They are defined not by the cities that were destroyed, but by the peaceful world they need and seek to build."

Nagasaki resident Kiyomi Fukushima, who was 5 years old at the time of the bombing, still recalls seeing black rain falling from the sky and feeling scared.

"We have enjoyed peace for the past decades and I hope it will continue. Somehow the world feels less safer and it's frustrating," said Fukushima, 79, after visiting the park early in the morning.

As the only nation to have come under nuclear attack, Japan has backed the creation of a nuclear free world, and Prime Minister Shinzo Abe has vowed to make strenuous efforts toward the goal.

"In recent years, differences in the approaches of various countries on nuclear disarmament have become evident," Abe said in his speech.

Japan will serve as an intermediary between nuclear powers and non-nuclear states, Abe said, adding that an accurate understanding of the tragic realities of the atomic bombings and cooperation from both parties is essential.

As of March, the number of hibakusha stood at 154,859, with their average age at 82.06.

"My mother used to come to the Peace Park every year for the ceremony but she can't anymore (due to her age)," said Nagasaki resident Naomi Maeda, 51.

She visited the park for her mother, who experienced the atomic bombing. "It's our turn to keep peace."