Several thousand people – including Prime Minister Shinzo Abe and survivors - observed a minute's silence in the center of the western Japanese city in memory of the victims at 8:15 am (UTC 2315), the time when a US B-29 bomber dropped the first atomic bomb on the city on August 6, 1945.

"Here in Hiroshima city, I renew my pledge to do my utmost to realize the nuclear free world and the eternal peace," Abe said at the ceremony. "I also pray for the peace of the A-bomb victims and the bereaved families, those attending here as well as the citizens of Hiroshima city."

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The US dropped a second atomic bomb on Nagasaki on the island of Kyushu on August 9, six days before Japan surrendered, ending World War II.

"This hell is not a thing of the past," Hiroshima Mayor Kazumi Matsui said at the ceremony. "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."

Matsui urged nuclear states, as well as Japan, to join the nuclear weapons ban treaty adopted by the UN in July. He also spoke of the threat posed by North Korea.

Two test-firings of Hwasong-14 inter-continental ballistic missiles recently suggest that major US cities such as Los Angeles and Chicago are within range of North Korean weapons.

Barack Obama went to Hiroshima in May 2016, the first incumbent US president to visit the site of the bombing, although did not issue an apology.

jbh/bw (Reuters, dpa)