The White House on Monday said it is still weighing a possible first presidential visit to Hiroshima, one of two Japanese cities bombed by a US nuclear weapon.

"We're obviously hard at work planning that trip," White House press secretary Josh Earnest said of a potential visit that could prove controversial.

"The president has been to Japan three or four times as president, and each time the president has traveled there, this question has come up and we've considered it each time."

Obama will visit central Japan in late May for a Group of Seven summit.

Last month, Secretary of State John Kerry became the highest ranking US political figure to visit Hiroshima.

He said he was "deeply moved" by the experience and a "gut-wrenching display that tugs at all your sensibilities as a human being."

"Everyone should visit Hiroshima, and everyone means everyone," he added, fueling speculation that Obama would go.

Japan has long urged world leaders to visit Hiroshima and Nagasaki to see the horrors of the atomic bombings and join efforts to eradicate nuclear arms.

But a presidential visit could rile Obama's opponents and some in the military whose predecessors carried out presidential orders to drop the bombs.

The visit would come at a particularly sensitive time. This December marks the 75th anniversary of Japan's attack on Pearl Harbor, in Obama's home state of Hawaii.

On August 6, 1945, the US dropped the world's first atomic bomb on Hiroshima, killing around 140,000 people, including those who survived the explosion itself but died soon after from severe radiation exposure.

Three days later, the US military dropped a plutonium bomb on the port city of Nagasaki, killing some 74,000 people.

The bombings are controversial in the United States, where opinion remains divided over whether their use in the closing days of World War II was justified.

Earnest said Obama did not believe the United States owes Japan an apology for that military action.