The imperial couple will mark the 40th anniversary of diplomatic ties between the two countries.

They will also visit Malaysia and Thailand as part of a visit seen as aiming to heal historical scars.

Emperor Akihito, 72, said that he hoped the visit would strengthen mutual understanding and friendship. He was speaking as he boarded the plane at Tokyo's Haneda airport, the BBC reported.

He also addressed the subject of the country's wartime history at a press conference.

"I repeatedly feel pain in my heart about the fact that many people, including the Japanese, lost their lives during the war," he said.

"We should never forget about this history," he said, adding he was deeply concerned that the younger generation lacked knowledge of the subject.

Japan invaded Singapore in 1942 and occupied it until 1945, its three-year rule marked by severe brutality in which thousands of civilians and allied prisoners died.

But the historical issue has not come to dominate the bilateral relationship, as it has with China, and ties between the two countries are cordial.

The imperial couple, who visited Singapore in 1970 and 1981 as Crown Prince and Princess, will attend a banquet hosted by Singaporean President S.R. Nathan, where the emperor is due to give a speech.

The couple will then visit Malaysia, before attending the celebrations in Bangkok to mark the Thai king's 60 years on the throne.