Nagasaki was flattened in an atomic inferno three days after Hiroshima -- twin nuclear attacks that rang in the nuclear age and gave Japan the bleak distinction of being the only country to be struck by atomic weapons.

Early Sunday, people attended a mass held in memory of victims at Urakami Church, near the site of the bombing, while others took part in a memorial service at the city's Peace Park.

The number of participants has been reduced to roughly one tenth the figure in previous years, with proceedings broadcast live online in Japanese and English.

Terumi Tanaka, 88, who survived the Nagasaki bombing when he was 13 at his house on a hillside, remembers the moment everything went white with a flash of light, and the aftermath.

"I saw many people with terrible burns and wounds evacuating ... people who were already dead in a primary school-turned shelter," Tanaka told AFP in a recent interview, saying his two aunts died.

Atomic bomb survivors "believe that the world must abandon nuclear arms because we never want younger generations to experience the same thing", he said.

Tanaka suspects people have become complacent, believing another nuclear weapon will not be used.

"Human beings possess about 13,000 nuclear bombs now. Our question is how on Earth are we allowing that?" he said.

"Do people think they will never be used at all? You never know, really you never know."

The remembrance ceremonies come as worries linger over the nuclear threat from North Korea and growing tensions between the US and China over issues including security and trade.

The US dropped the first atomic bomb on Hiroshima on August 6, 1945, killing around 140,000 people. The toll includes those who survived the explosion itself but died soon after from radiation exposure.

Three days later, the US dropped a plutonium bomb on the port city of Nagasaki, killing 74,000 people.

Japan announced its surrender in World War II on August 15, 1945.

The United States has never acceded to demands in Japan for an apology for the loss of innocent lives in the atomic bombings, which many Western historians believe were necessary to bring a quick end to the war and avoid a land invasion that could have been even more costly.

Others see the attacks as unnecessary and even experimental atrocities.

Last year, Pope Francis met with several survivors on visits to Hiroshima and Nagasaki, paying tribute to the "unspeakable horror" suffered by the victims.

In 2016, Barack Obama became the first sitting US president to visit Hiroshima. He offered no apology for the attack but embraced survivors and called for a world free of nuclear weapons.