Japan's Emperor Akihito today said his advancing age and weakening health mean he may no longer be able to carry out his duties, setting the stage for Japan to prepare for an historic abdication.

"There are times when I feel various constraints such as in my physical fitness," the 82-year-old said in a national address.

"As we are in the midst of a rapidly ageing society, I would like to talk to you today about what would be a desirable role of the emperor in a time when the emperor, too, becomes advanced in age," he said.

Speculation about the emperor's future emerged last month with reports he had told confidantes that he would like to step down in a few years, in what would be the first abdication from the Chrysanthemum Throne in two centuries.

"I am worried that it may become difficult for me to carry out my duties as the symbol of the state with my whole being as I have done until now," he said, wearing a dark suit and sitting at a table in the pre-recorded video.

Akihito spoke obliquely -- never mentioning the word abdication and stressing he is legally prevented from commenting on the imperial system -- but analysts said his intention was clear.

Japanese Emperor Akihito sits on the throne at the upper house before opening the extraordinary Diet session at the National Diet in Tokyo on August 1, 2016. Photo: TORU YAMANAKA/AFP.

The comments will now allow the government to begin creating the necessary legal mechanism for a royal departure, which currently does not exist.

Prime Minister Shinzo Abe, in a swift response to the emperor's speech, said the government would take the emperor's remarks "seriously".

"Considering the emperor's duties, as well as his age and the burden (of the job), we have to firmly look at what we can do," he said.

'Concretely consider'

"The emperor did not use the word abdication, but his message clearly called on the public to concretely consider the way for that in the future," said Tomitaro Hashimoto, an assistant professor at Reitaku University.

"Legally, he can't request a revision of law," said Hashimoto, an expert on Japan's imperial system. "That's why he can't ask directly."

The address marked only the second time for Akihito to speak directly to the nation. The first was in the days after the March 2011 triple earthquake, tsunami and nuclear disaster as he sought to calm a nation undergoing its worst crisis since the war.

Japan's imperial house is said to be the world's oldest hereditary monarchy, and according to legend stretches back some 2,600 years in an unbroken line.

It is deeply ingrained in the nation's native Shinto religion and it comes with numerous ritual duties, including planting rice in a field within the palace grounds.

The speech comes during an annual time of sensitivity in Japan with August being a month of remembrance. Japan commemorated the US atomic bombing of Hiroshima on Saturday and does so again on Tuesday for Nagasaki.

Japan royal dynasty. Image taken from AFP

And next Monday, Japan pauses to recall the 71st anniversary of its defeat in World War II, an annual event at which the emperor delivers a speech.

Akihito has keenly embraced the role of symbol of the state imposed after the war. He is credited with seeking reconciliation both at home and abroad over the legacy of the war fought in his father's name.

He has ventured to a number of locales that saw intense fighting, including Okinawa at home and Saipan, Palau and the Philippines abroad, making sure to offer prayers for the souls of all the dead and not just Japanese.

Any eventual move by Akihito to step down appears to have wide public support.

A survey by Kyodo News last week showed that 85.7 percent of people surveyed were in favour of legal changes that would allow abdication.