Japan's Emperor Naruhito has said his wife Empress Masako "still has ups and downs" as she battles to recover from a longstanding stress-induced illness related to her royal duties.

The health of Masako, who was fit enough to attend all of Naruhito's ceremonies when he was sworn in as emperor last year, is a major talking point in Japan.

Hopes were raised that she had recovered from her illness as she was present at a series of public events last year but Naruhito warned the empress was "still on the path to recovery".