Japanese Emperor Akihito said Monday he is concerned his weakening health may make it hard to fulfil his duties, in a speech seen as signalling a possible future abdication.

"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 in an address to the nation.

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

Akihito spoke obliquely -- never mentioning the word abdication -- but the government is expected to interpret his comments as meaning his wish is to eventually step down.

It can then begin creating the necessary legal mechanism 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."

Speculation about the emperor's future emerged last month with reports he had told confidantes that advancing age was making it harder to perform his ceremonial duties and that he would like to step down in a few years.

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.

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.