Prime Minister Shinzo Abe led commemorations at the Peace Memorial Park in Hiroshima as residents offered silent prayers, lit candles and placed wreathes to remember those killed in the August 6, 1945 bombing.

And mayor Kazumi Matsui used the occasion to push the Abe administration to sign the UN treaty on the prohibition of nuclear weapons (TPNW), approved by more than 120 nations, but rejected by the US and other nuclear armed countries.

"I call on the government of the only country to experience a nuclear weapon in war to accede to the hibakusha's (atomic bomb victims) request that the TPNW be signed and ratified," said Matsui.

"I urge Japan's leaders to manifest the pacifism of the Japanese constitution by displaying leadership in taking the next step towards a world free from nuclear weapons."

The mayor urged world leaders to come to the city to see the memorial for themselves, amid expectations that Pope Francis will visit later this year.

The ongoing threat from atomic weapons in the region was demonstrated as nuclear-armed North Korea fired off two "unidentified projectiles" off its eastern coast -- the latest in a series of tests.

Meanwhile, the US and Russia failed to renew a Cold War-era nuclear pact last week, triggering renewed fears of an arms race.

Japan remains the only country to have experienced atomic attack -- against Hiroshima and Nagasaki, days ahead of the country's surrender on August 15, 1945 to end World War II.

For his part, Abe reiterated a pledge to serve as a "bridge" between nations with and without nuclear weapons to rid the world of them.

"With help from both sides, I will patiently encourage them to engage in dialogue and I am resolved to lead international efforts towards this end," he said.