Japan on Thursday marks 75 years since the world's first atomic bomb attack, with the coronavirus pandemic forcing a scaling back of ceremonies to commemorate the victims.

Survivors, relatives and a handful of foreign dignitaries will attend this year's main event in Hiroshima to pray for the victims and call for world peace. 

But the general public will be kept away, with the ceremony instead broadcast online.

Other events, including a gathering to float lanterns along the Motoyasu River, have been cancelled as coronavirus cases spike in parts of Japan.

The annual commemoration is "Hiroshima's mission of calling on people across the world to work towards peace", mayor Kazumi Matsui told reporters. 

Participants will offer a silent prayer at 8:15 am (2315 GMT Wednesday), the exact time the first nuclear weapon deployed in wartime hit the city.

Around 140,000 people were killed, many of them instantly, with others perishing in the weeks and months that followed, suffering radiation sickness, devastating burns and other injuries.

Three days later the United States dropped a second atomic bomb on Nagasaki, where 74,000 people were killed.

- 'Unspeakable horror' -

The historical assessment of the bombings remains subject to some controversy. The United States has never apologised for the bombings, which many in the US see as having ended the war.

Japan announced its surrender just days later on August 15, 1945, and some historians argue the bombings ultimately saved lives by avoiding a land invasion that might have been significantly more deadly.

But in Japan, the attacks are widely regarded as war crimes because they targeted civilians indiscriminately and caused unprecedented destruction.

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

Hiroshima and Nagasaki were key stops on Pope Francis's first trip to Japan last year, where he denounced the "unspeakable horror" of the attacks.

At Thursday's ceremony, Hiroshima's mayor and a representative of bereaved families will deliver remarks in front of a cenotaph inscribed with the names of victims.

Volunteers will then livestream a tour of buildings affected by the bombing, and share testimonies by two atomic bomb survivors as part of efforts to mark the anniversary despite the virus.

The pandemic stalking the globe carries an all-too-familiar fear for some survivors, including 83-year-old Keiko Ogura, who lived through the Hiroshima bombing.

With the outbreak of the virus, "I recall the fear I felt right after the bombing," she told journalists last month. 

"No one can escape."

- 'Solidarity among mankind' -

The global nature of the threat requires a global solution, she said.

"Whether it's the coronavirus or nuclear weapons, the way to overcome it is through solidarity among mankind."

The landmark anniversary this year underscores the dwindling number of bomb survivors, known in Japan as "hibakusha".

Those who remain were mostly infants or young children at the time, and their work to keep the memory of the bombings alive and call for a ban on nuclear weapons has taken on increasing urgency as they age.

Activists and survivors have created archives of everything from the recorded testimony of hibakusha to their poems and drawings.

But many fear interest in the bombings is fading as they recede beyond the horizon of lived experience and into history.

"Just storing a pile of records... is meaningless," said Kazuhisa Ito, the secretary general of the Hibakusha Assembly of Memory Heritage, an NGO that compiles records and documents from survivors.

"What we want is to engage young people with this issue and exchange views with them, globally," he told AFP.