"Humanity is playing with a loaded gun" as crises with the potential for nuclear disaster proliferate worldwide, UN head Antonio Guterres said in Hiroshima on Saturday, the 77th anniversary of the first atomic bomb attack.

At an annual memorial, Guterres warned of the risk posed by crises in Ukraine, the Middle East and the Korean peninsula as he described the horrors endured by the Japanese city.

"Tens of thousands of people were killed in this city in the blink of an eye. Women, children and men were incinerated in a hellish fire," he said.

Survivors were "cursed with a radioactive legacy" of cancer and other health problems.

"We must ask: What have we learned from the mushroom cloud that swelled above this city?"

Around 140,000 people died when Hiroshima was bombed by the United States on August 6, 1945, a toll that includes those who perished after the blast from radiation exposure.

Today, "crises with grave nuclear undertones are spreading fast," Guterres said, repeating warnings he made this week at a nuclear Non-Proliferation Treaty conference in New York.

"Humanity is playing with a loaded gun."

Before dawn, survivors and their relatives began to gather at Hiroshima's Peace Memorial Park to pay tribute to the victims.

A silent prayer was held at 8.15 am, the moment the bomb was dropped.

The Russian ambassador was not invited to the ceremony but visited Hiroshima on Thursday to lay flowers at the memorial site.

Since Russia invaded Ukraine in February, President Vladimir Putin has made thinly veiled threats hinting at a willingness to deploy tactical nuclear weapons.

In a speech on Saturday, Hiroshima Mayor Kazumi Matsui cited Leo Tolstoy, the Russian author of "War and Peace", saying: "Never build your happiness on the misfortune of others, for only in their happiness can you find your own."

Three days after the Hiroshima bombing, Washington dropped a second atomic bomb on the Japanese port city of Nagasaki, killing about 74,000 people and leading to the end of World War II.

There are now fewer than 119,000 officially recognised survivors of the two nuclear attacks, according to government statistics from March.

The United States remains the only country ever to have used nuclear weapons in conflict.

But around 13,000 are now held in state arsenals worldwide, Guterres said.

Saturday was the first time Guterres attended the Hiroshima memorial in person as UN chief, with a visit last year cancelled because of the Covid-19 pandemic.