"Any suggestion by any candidate for high public office that we should be building more weapons and giving them to a country like Korea or Japan are absurd on their face and run counter to everything that every president, Republican or Democrat alike, has tried to achieve ever since World War II," he told the press at a news conference.
Kerry is the first sitting secretary of state to visit the revered memorial to the nuclear bombing of Hiroshima.
Kerry, E.U. Foreign Policy Chief Federica Mogherini and other foreign ministers from the Group of Seven, who are in Japan
for two days of talks, visited the site of the world's first use of an atomic weapon in warfare earlier Monday.
"It is a gut-wrenching display. It tugs at all of your sensibilities as a human being," Kerry said of the memorial.
"It reminds everybody of the extraordinary complexity of choices of war and of what war does to people, to communities, to countries, to the world. This was a display that I will personally never forget."
Their trip was a stark reminder that looming over the current round of G7 talks is the history of the city they're being held in. It also came a day after a State Department official said
Kerry won't apologize for the twin bombings of Hiroshima and Nagasaki.
Hiroshima was devastated when the United States dropped an atomic bomb on the city in August 1945. The bomb, which led to the end of World War II
, killed 140,000 people -- either initially from the impact or later due to radiation exposure.
Three days later, the U.S. dropped a second bomb on the port city of Nagasaki, killing about 70,000 people.
'Everyone in the world' should see this
Under tight security, the diplomats laid wreaths at Peace Memorial Park then walked to the Atomic Bomb Dome. The ruin, which is the only structure left standing near the bomb's hypocenter, serves as a memorial to the people killed in the bombing.
Earlier, they toured the peace museum documenting the devastating impact of the attack. The museum houses victims' and survivors' burnt clothing and personal effects.
Together, these three sites stand as a powerful symbol for nuclear disarmament.
"Everyone in the world should see and feel the power of this memorial. 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," Kerry wrote in a message in the site's guestbook.
"War must be the last resort -- never the first choice. This memorial compels us all to redouble our efforts to change the world, to find peace and build the future so yearned for by citizens everywhere."
Despite the message's tone, a senior State Department official traveling with Kerry said emphatically that America's top diplomat will not issue an apology for its use of nuclear weapons and the devastation it caused.
"If you are asking whether the secretary of state came to Hiroshima to apologize, the answer is no," the official said. "If you are asking whether the secretary -- and I think all Americans and all Japanese -- are filled with sorrow at the tragedies that befell so many of our countrymen, the answer is yes."
Before he visited the memorial, Kerry spoke with Fumio Kishida, the Japanese foreign minister.
"My visit to Hiroshima has very special meaning about the strength of our relationship and the journey we have traveled together since the difficult time of the war," Kerry said.
"We will revisit the past and honor those who perished, (but) this trip is not about the past; it's about the present and the future."
More on the agenda
The Japanese government and public hope the location of the summit in Hiroshima will foster better understanding among nations about Japan's staunch nuclear stance.
Kashida also addressed Trump's proposal that Japan and South Korea arm themselves with nuclear weapons, which has raised eyebrows in Japan
"For us to obtain nuclear weapons is completely inconceivable," Kishida said Monday.
But Kerry has much more than his visit to the Hiroshima memorial on the agenda.
During the first day of talks, the G7 -- made up of the United States, Britain, Canada, France, Germany, Italy and Japan -- discussed the Syrian civil war
, the migrant crisis facing Europe and violence in Ukraine
, aides traveling with Kerry said.
Kerry also led a discussion about the political chaos in Iraq and Afghanistan, after having visited both countries this week. The fight against ISIS
also dominated the discussions. Some of the European members are grappling with security challenges after recent terrorist attacks in Brussels and Paris
But Japan hopes to highlight pressing concerns in Asia, including China's activities in the South China Sea and the North Korean nuclear threat.
Kerry stressed why he believes the G7 and and international cooperation are important.
"The peaceful stable international system that we have built in the decades since WWII are not a given," he said. "They're not automatic. They require work, investment, leadership."
Kerry's trip to Hiroshima could also pave the way for President Barack Obama
to visit the city next month, when he travels to Japan for the G7 leaders summit, fueling speculation of whether he may issue a formal apology for the bombing and the devastation it caused.
The senior State Department official said the White House had not made a formal decision.