A bell tolled Monday in Hiroshima as Japan marked 73 years since the world's first atomic bombing, with the city's mayor warning that rising nationalism worldwide threatened peace.

The skies over Hiroshima's Peace Memorial Park were clear, just as they were on August 6, 1945, when an American B-29 bomber dropped its deadly payload on the port city dotted with military installations, ultimately killing 140,000 people.

About 50,000 people, including Hiroshima residents and representatives from 80 countries, including U.S. Ambassador William Hagerty, attended this year's ceremony.

Survivors, their relatives and other participants marked the 8:15 a.m. blast with a minute of silence.

Hiroshima Mayor Kazumi Matsui, standing at the park near ground zero for the annual ceremony, opened his speech by describing the hellish scene of the blast that morning 73 years ago and the agony of the victims, telling the audience to listen "as if you and your loved ones were there." Then he raised concerns about the global rise of nationalism, egocentrism and tensions, and urged Japan's government to take more leadership toward achieving a truly nuclear-free world.

Without naming specific nations, Matsui warned that "certain countries are explicitly expressing self-centered nationalism and modernizing their nuclear arsenals."

They were "rekindling tensions that had eased with the end of the Cold War", he added.

Nuclear deterrence and nuclear umbrellas are "inherently unstable and extremely dangerous" approaches that seek to maintain international order by only generating fear in rival countries, he said, urging world leaders to negotiate in good faith to eliminate nuclear arsenals instead in a year when President Donald Trump pledged to increase the U.S. nuclear arsenal.

"If the human family forgets history or stops confronting it, we could again commit a terrible error. That is precisely why we must continue talking about Hiroshima," Matsui said.

"Efforts to eliminate nuclear weapons must continue."

Matsui said in his speech that Japan's government should do more to achieve a nuclear-free world by helping the Treaty on the Prohibition of Nuclear Weapons take effect.

The Treaty on the Prohibition of Nuclear Weapons was approved by the United Nations in July last year and the group played a key role in campaigning for the treaty, the International Campaign to Abolish Nuclear Weapons (ICAN), received the 2017 Nobel Peace Prize.

Matsui's call however highlighted Japan's contradictory relationship with nuclear weapons although Japanese officials routinely argue that they oppose atomic weapons.

Japan, which hosts U.S. troops and is covered by the U.S. nuclear umbrella protecting it from attack, has not signed the treaty. A survey conducted by the Kyodo News agency showed 81 percent of the survivors want Japan to sign the Treaty on the Prohibition of Nuclear Weapons.

Japan should live up to the spirit of its pacifist constitution to lead the international community "toward dialogue and cooperation for a world without nuclear weapons," Matsui said.

The anniversary comes amid hopes to denuclearize North Korea after President Donald Trump and North Korean leader Kim Jong Un made vague aspirational statements of denuclearizing the peninsula when they met in Singapore in June. "We in civil society fervently hope that the easing of tensions on the Korean Peninsula will proceed through peaceable dialogue," Matsui said.

Japan has largely maintained a hard line on Pyongyang, in particular pushing for movement on citizens abducted decades ago by North Korean agents.

But reports suggest Tokyo is considering a summit soon between Kim and Prime Minister Shinzo Abe, with local media floating a possible meeting on the sidelines of an international forum in Russia's Vladivostok next month.

"Ultimately, I myself will have to directly face chairman Kim Jong Un and engage in dialogue and resolve the nuclear, missile and, above all, the all-important abduction issue, and then build new Japan-North Korea relations," Abe said in Hiroshima on Monday.

Abe, whose government has chosen not to participate in the UN Treaty on the Prohibition of Nuclear Weapons, said Japan had a responsibility to bridge the gap between nuclear and non-nuclear nations.

"In recent years, it has become evident that gaps exist among countries about ways to proceed with nuclear arms reduction," Abe told the ceremony, without directly referring to the treaty.

"Our nation, while maintaining our (non-nuclear weapons) principles, will patiently work to serve as a bridge between the two sides and lead efforts by the international community" to reduce nuclear weapons, Abe said.

Japan suffered two nuclear attacks by the United States at the end of World War II -- first in Hiroshima and then in Nagasaki three days later.

The attack on Hiroshima, 700 kilometres west of Tokyo, in the closing days of World War II ushered in the nuclear age. It was the first use of nuclear weapons against human beings.

The bombings claimed the lives of 140,000 people in Hiroshima and 74,000 people in Nagasaki.

Barack Obama became the first sitting U.S. president to visit Hiroshima in May 2016.