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 -- the Middle East, to the Korean peninsula, to Russia's invasion of Ukraine", 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 offer flowers and prayers.

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 plutonium bomb on the Japanese port city of Nagasaki, killing about 74,000 people and leading to the end of World War II.

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, the UN head said.

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