A speech and a Nobel prize have raised hopes in Japan that Barack Obama will become the first sitting American president to visit Hiroshima or Nagasaki, the two cities devastated by U.S. atomic bombs in World War II.

Past presidents have avoided a visit that could raise controversy at home, and U.S. officials say it is highly unlikely Obama will travel to either city during a two-day stop in Tokyo next month.

On Tuesday, the mayors of Hiroshima and Nagasaki went to the U.S. Embassy in Tokyo to formally invite Obama to their cities before a U.N. review of the Nuclear Nonproliferation Treaty next May.