John Kerry has become the first US secretary of state to pay his respects at Hiroshima's memorial to victims of the 1945 US nuclear attack.
Following the visit by Mr Kerry and his counterparts from the Group of Seven advanced economies, the ministers issued a statement reaffirming their commitment to building a world without nuclear arms.
However, they said the push had been made more complex by North Korea's repeated provocations and by the worsening situations in Syria and Ukraine.
Mr Kerry toured the Hiroshima Peace Memorial and Museum, whose haunting displays include photographs of badly burned victims, the tattered and stained clothes they wore and statues depicting them with flesh melting from their limbs.
The ministers from Britain, Canada, France, Germany, Italy, Japan and the United States then laid white wreaths at a cenotaph to the victims of the 6 August 1945 bombing by the United States, which reduced the city to ashes and killed 140,000 people by the end of that year.
"Everyone in the world should see and feel the power of this memorial," Mr Kerry wrote in a guest book.
"It is a stark, harsh, compelling reminder not only of our obligation to end the threat of nuclear weapons, but to rededicate all our effort to avoid war itself."
After a moment of silence by the ministers, Japanese schoolchildren, who had lined the entrance waving flags of all the G7 nations, presented them with leis made of paper cranes, symbolising peace, in each country's national colours.
At Mr Kerry's suggestion, the ministers also made an impromptu visit to the Atomic Bomb Dome, the skeletal remains of the only structure left standing near the hypocentre of the bomb explosion and now a UNESCO World Heritage site.
Three days after a US warplane dropped a nuclear bomb on Hiroshima, another atomic bomb was dropped on Nagasaki.
Japan surrendered six days later.
Mr Kerry's trip could pave the way for an unprecedented visit to Hiroshima by a sitting US president when Barack Obama attends the annual G7 leaders summit in another Japanese city next month.
A visit could be controversial in America if it were viewed as an apology.
A majority of Americans still view the bombings as justified to end the war and save US lives, while the vast majority of Japanese believe it was not justified.
In a separate, detailed statement, the G7 ministers singled out North Korea for sharp criticism, condemning its recent nuclear test and launches using ballistic missile technology.
And in a statement on maritime security, they voiced their strong opposition to provocative attempts to change the status quo in the East and South China Seas, an apparent reference to China, which is locked in territorial disputes with other nations including the Philippines, Vietnam and Japan.