The Obama administration said Wednesday it is sending its ambassador in Tokyo to a ceremony next week marking the atomic bombing of Hiroshima, the first time a US ambassador has attended the event.

"Ambassador John Roos will represent the United States at the August 6 Hiroshima Peace Memorial, to express respect for all of the victims of World War II," State Department spokesman Philip Crowley told reporters.

He said Roos is the first ambassador to attend the event, but could not immediately confirm if he is the highest-ranking US official or the only US official so far to join the ceremony.

Roos is expected to lay a floral wreath at the Hiroshima Peace Memorial on August 6, the 65th anniversary of the World War II bombing that helped force Japan's surrender, reports said.

Japan is the only nation to have been attacked with atomic bombs.

More than 140,000 people were killed instantly in Hiroshima or died in the days and weeks after the US attack. Three days later a US plane dropped a second atomic bomb on Nagasaki, killing more than 70,000 people.