Antonio Guterres, the first sitting UN secretary general to attend ceremonies honouring those who died in the bombing, said that even 73 years later, there was still a "shadow cast by the dread of unthinkable carnage."

Nuclear-armed states are spending "vast sums" to modernise their arsenals and "disarmament processes have slowed and even come to a halt," noted Guterres.

"There is an urgent need for disarmament of all kinds, but especially nuclear disarmament," stressed the secretary general.

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 severe radiation exposure.

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

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

On Thursday in Nagasaki, crowds flocked to pray and pay their respects at the 10-metre (33-foot) high peace statue.

Guterres and Japanese Prime Minister Shinzo Abe laid wreaths to honour the dead and family members carrying buckets of water to purify altars bowed in memory of their loved ones.

The ceremony came amid lingering worries over North Korea's nuclear threat and in a year when President Donald Trump has pledged to bolster the US arsenal.

Mayor of Nagasaki Tomihisa Taue also issued a passionate call for denuclearisation.

"To the great concern of those in the atomic-bombed cities, a shift towards openly asserting that nuclear weapons are necessary and that their use could lead to increased military might is once again on the rise," Taue said.