John Kerry on Monday became the first US secretary of state to visit Hiroshima's atomic bomb memorial, calling it a "stark, harsh, compelling reminder" of the need to rid the world of nuclear weapons.

Kerry, who was accompanied by other G7 foreign ministers, is the highest-ranking US administration official to pay his respects at the spot in Japan where American planes launched the first ever nuclear attack, more than 70 years ago.

His trip comes as White House officials say President Barack Obama is considering a stop in the now-bustling Japanese city late next month around the time of a Group of Seven summit, which is being held in another part of the country.

Kerry's visit, and speculation that Obama may also go to Hiroshima, prompted some suggestion that Washington might make an official apology over the August 1945 bombing, which killed 140,000 people.

But America's top diplomat was quick to douse that speculation, and a State Department official flatly ruled out an apology.

"We will revisit the past and honour those who perished, (but) this trip is not about the past," Kerry told Japanese Foreign Minister Fumio Kishida earlier Monday.

"It's about the present and the future."

Arriving under tight security, the G7 ministers and the foreign policy chief of the European Union started their visit Monday at a museum that shows the devastating impact of the bombing -- such as survivors' burned clothing and other personal effects.

"Everyone in the world should see and feel the power of this memorial," Kerry wrote in the museum's 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.

"War must be the last resort -- never the first choice."

The G7 later issued its Hiroshima Declaration that called for a "world without nuclear weapons", as it noted the "immense devastation and human suffering" caused by the wartime atom bombings in Hiroshima and Nagasaki.

- 'No bad feelings' -

Hundreds of schoolchildren waved G7 nation and EU flags as the group walked to a cenotaph in the leafy park next to the museum.

Kerry and his counterparts laid wreaths at the site, with the ruins of a domed building gutted by the blast in the background.

Later children presented them with necklaces made of paper cranes -- a symbol of peace -- woven in the bright colours of their national flags.

Hiroshima businessman Jun Miura said he hoped Obama would make a trip to the city of 1.2 million next month.

"There are no bad feelings and we're not angry," the 43-year-old told AFP as he watched the event.

"I want the president to see for himself exactly what happened. I am sure he has seen video of it, and read about it. But you have to come here to see it and contemplate."

For US tourist Jeremy Griffiths, visiting the memorial is a stark reminder of the scale of the damage.

"You can read all you want, but until you are actually at the place (where) it had occurred -- it just changes how you look at it," said the 29-year-old IT programmer from Florida.

Many were killed instantly when the bomb was dropped, creating a firestorm that flattened swathes of the city. Thousands of others died later from radiation exposure.

Three days later, on August 9, 1945, another US atomic bomb exploded over the city of Nagasaki, killing some 74,000 people. On August 15, Emperor Hirohito announced Japan's surrender, ending World War II.

The issue of the bombings is a highly emotive one in both Japan and the United States.

Japan, as the only nation to have experienced a nuclear attack, emphasises the suffering its people endured. But while publicly calling for the eradication of nuclear weapons, it has for decades been a close security ally of Washington under the protection of the US nuclear umbrella.

Many in the US, meanwhile, chafe at the suggestion of an apology, saying that Japan started the war with its attack on Pearl Harbor and argue that the bombings hastened the war's end, and prevented more casualties.

Monday's visit came as the ministers wrapped up their final day of meetings with discussions focused on global hotspot issues including the fight against Islamic State group and other security threats such as North Korea.

Instability in the Middle East and elsewhere was also on the agenda.