Nagasaki on Sunday marked the 75th anniversary of the U.S. atomic bombing of the city in a ceremony held at the grandiose Peace Statue, with the mayor and survivors urging world leaders to work for a ban on nuclear weapons.

At 11:02 a.m. local time, Nagasaki survivors and others stood in a minute of silence to honor more than 70,000 people who died when the B-29 bomber Bockscar dropped a 4.5-ton plutonium-239 bomb dubbed "Fat Man" on the city.

"As a country that has experienced the horrors of nuclear weapons, please sign the Treaty on the Prohibition of Nuclear Weapons and see to its ratification at the earliest possible date," said Tomihisa Taue, Nagasaki mayor. "In addition, please examine the plan to establish a nuclear-weapon-free zone in Northeast Asia. Please adhere for eternity to the peaceful principles of the Japanese constitution, which includes the determination not to wage war."

Prime Minister Shinzo Abe laid a wreath at the memorial to remember the victims and made a speech but avoided any direct reference to the treaty.

"The tragedy in Nagasaki and Hiroshima and the suffering caused to its people must never be repeated again," Abe said. "As the only country that had experienced nuclear weapons during war, it remains the unchanged mission of our nation to firmly move forward step by step the efforts of the international community towards realizing a world without nuclear weapons."

Japan has not signed the treaty. Many survivors developed cancer or other illnesses because of radioactive contamination.

U.N. Undersecretary-General and High Representative for Disarmament Affairs Izumi Nakamitsu said in a message to the Nagasaki Peace Memorial that the world "must return to the understanding that a nuclear war cannot be won and must never be fought," adding that "there is an urgent need to stop the erosion of the nuclear order. All countries possessing nuclear weapons have an obligation to lead."