Hiroshima: John Kerry will not apologise for the US atomic bombing of Hiroshima during his visit to the city for weekend G7 talks, a State Department official said on Sunday.

Speculation has been building that Kerry's planned visit to a Hiroshima blast memorial tomorrow could see the US secretary of state issue a first-ever apology for the wartime attack.

But a State Department official said a formal apology from America's top diplomat was not on the cards.

"If you are asking whether the secretary of state came to Hiroshima to apologise, the answer is no," the official, who asked not to be named, told reporters travelling with Kerry.

"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."

Kerry is the first secretary of state to visit the Japanese city, which was obliterated by a US nuclear bomb on August 6, 1945 that killed 140,000 people, including those who died afterwards from severe radiation exposure.

Three days later another blast killed some 74,000 people in Nagasaki, in the closing chapters of World War II.

Kerry's landmark trip is widely seen as paving the way for Barack Obama to possibly become the first serving US president to visit the city, when he heads to Japan next month for a Group of Seven summit.

Kerry and other G7 foreign ministers are meeting this weekend to discuss a range of global hotspot issues, including the Middle East, the refugee crisis, the conflict in Ukraine and global terrorism.