Hiroshima's appeal of "never again" on the anniversary of the world's first atomic bomb attack has acquired renewed urgency as North Korea moves ever closer to acquiring nuclear weapons.

The city marked the 72nd anniversary of the August 6, 1945, attack that killed 140,000 people with a sombre ceremony on Sunday, a minute of silence to remember the dead and a call to eliminate nuclear weapons.

Mayor Kazumi Matsui said that "hell is not a thing of the past," urging that the "absolute evil" of atomic weaponry be banished.

North Korea's efforts to acquire nuclear weapons and its recent test-firings of intercontinental ballistic missiles have brought deep unease in Hiroshima, even as many Japanese appear resigned to the growing threat.