Japanese Emperor Akihito, 77, has a heart problem and will undergo additional medical checks this week, the Imperial Household Agency said on Wednesday.
Akihito, who ascended the throne after the death of his father Hirohito in 1989, is known for opening up the closed royal family to the public and for trying to mend Japan's ties with Asian neighbours who suffered under Japanese militarism during World War Two.
Doctors found that Akihito, who had surgery for prostate cancer in 2003, was not getting an adequate supply of blood to the heart following physical exertion, a spokesman for the royal agency said.
Doctors will conduct further checks on Friday to determine what treatment may be required, the spokesman said. He gave no further details.
The emperor had no obvious symptoms and would carry out official duties as scheduled, Kyodo news agency reported.
Akihito suffered stress-related health issues in late 2008, including irregular pulse and stomach bleeding.
The following year, the royal agency said he would cut back on official duties such as speeches and meeting foreign dignitaries.
Television footage on Wednesday showed the emperor meeting visiting Uzbek President Islam Karimov. The two were smiling and chatting while accompanied by their wives.
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