"Needless to say, it's a very, very powerful reminder of not just the impact of war in a lasting way on people, on countries, but it also underscores the importance of the agreement we reached with Iran to reduce the possibility of more nuclear weapons," Kerry said on the sidelines of a regional diplomatic gathering in Kuala Lumpur.
"And the United States and other countries are working to move to - particularly Russia (and) the United States, with our agreement - to reduce the number of existing nuclear weapons."
But the deal has run into some opposition in the US Congress, which is due to vote on it.
Tens of thousands of people gathered today in Hiroshima to commemorate the atomic bombing that helped end World War II but still divides opinion today over whether the total destruction it caused was justified.
Bells tolled as a solemn crowd observed a moment of silence at 8:15 am, when the detonation turned the western Japanese city into an inferno, killing thousands instantly and leaving others to die a slow death with horrible injuries.