Some Japanese media, including Jiji Press news agency and the Mainichi newspaper, said it was the first time the 81-year-old used those words at an annual memorial on the day Japan surrendered to the Allies in 1945.
"Recalling the past with profound remorse over the previous war and sincerely hoping that the tragedy of war is never repeated, I, together with people across the nation, express my heartfelt sorrow toward those who fell in battle, and pray for the further development of our country and world peace," he said in a speech in Tokyo.
In a move likely to further strain relations, a pair of cabinet ministers visited the controversial Yasukuni shrine, which neighbouring countries see as an ugly symbol of Tokyo's militarist past.
Akihito, a largely symbolic figurehead, has never visited the shrine.