The fear of another nuclear bomb attack is growing on the 72nd anniversary of the US atomic bombing of Nagasaki, Japan, the city's mayor said, apparently referring to the North Korean nuclear threat in the region.

Speaking at a ceremony marking the nuclear tragedy of the World War II on Wednesday, Tomihisa Taue urged nuclear states to abandon such weapons and criticised Japan's government for not taking part in the global efforts towards a nuclear ban.

The bombing anniversary comes just as Pyongyang and Washington are trading escalating threats over the former's nuclear weapons programme.

President Donald Trump threatened North Korea "with fire and fury", and North Korea's military said it was examining its plans for attacking Guam.

RELATED: Japan marks 72 years since Hiroshima atomic bomb

"The international situation surrounding nuclear weapons is becoming increasingly tense," Taue said at Nagasaki's Peace Park.

"A strong sense of anxiety is spreading across the globe that in the not too distant future these weapons could actually be used again."

The world's first atomic bomb, used on August 6, 1945, killed 140,000 people in Hiroshima. The bombing of Nagasaki three days later killed 70,000 more.

At 11:02am, the time that the bomb struck 72 years ago, people at the ceremony observed a moment of silence as the peace bell rang.

"The nuclear threat will not end as long as nations continue to claim that nuclear weapons are essential for their national security," Taue said.

Abe government criticised

Taue also criticised Prime Minister Shinzo Abe's government for what he said were empty promises about working to achieve a nuclear-free world.

He said Japan's absence even during diplomatic negotiations for the UN Nuclear Prohibition Treaty, adopted in July, is "incomprehensible to those of us living in the cities that suffered atomic bombings".

The outspoken mayor praised the atomic bombing survivors, or "hibakusha", for their lifelong devotion to the effort.

He urged Japan's government to change its policy of relying on the US nuclear umbrella and join the nuclear prohibition treaty as soon as possible.

Abe, in a speech that was almost a repeat of what he said in Hiroshima, did not mention the UN nuclear ban treaty.

More than 175,000 hibakusha have died in Nagasaki since the attack, including 3,551 in the past year, while over 300,000 of their peers have died in Hiroshima.

The average age of the survivors is more than 81 years. Many suffer from long lasting effects of radiation.