TOKYO (AP) - Japanese Empress Masako, still recovering from stress-induced mental health issues, said on Monday (Dec 9) that she was happy to have completed her duties as part of Emperor Naruhito's enthronement rituals, and pledged to keep up the work and help her husband more for the happiness of the people.

In a palace statement marking her 56th birthday, Empress Masako thanked people who had warmly welcomed the couple after Emperor Naruhito succeeded to the throne on May 1, following his father's abdication.

"Many smiling faces I've seen in many places are precious memories for me and they will be my big moral support as I move forward," the statement said.

A Harvard-educated former diplomat, Empress Masako had been largely absent from public appearances for years.

She developed adjustment disorder, a condition marked by depression and other stress-induced symptoms, after giving birth to the couple's only child, Princess Aiko, and facing pressure to have a son to continue Japan's male-only imperial succession.

Emperor Naruhito's succession rituals spanned from late April to early December, and Empress Masako was seen smiling and seemed healthy at her public appearances.

Her doctors welcomed her accomplishment as a positive sign, but cautioned the people against raising their expectations too high, saying that could interfere with her recovery.

Empress Masako's long absence from imperial events and trips had raised concerns whether she could do even part of the work done by hugely popular former Empress Michiko.

But she accompanied Emperor Naruhito at all events, including his first public greeting as emperor, which some 140,000 people attended.

She sat next to Emperor Naruhito in an open car during a royal parade in November, enthusiastically waving to 119,000 well-wishers on the roadside. She appeared overwhelmed with emotion, wiping her tears with a handkerchief.

Empress Masako thanked Emperor Naruhito for his consideration and support for her and said that she hoped to further improve her health so she can give him more support.

"I hope to fulfil my duty as Empress, while trying to further improve my health so that I can help His Majesty and work for the people's happiness, together with him," the statement released by the Imperial Household Agency said.

Her doctors said Empress Masako has been able to expand her activities and regained confidence little by little as she constantly sought ways to maintain her health while taking care of her daughter, Princess Aiko. The people's warm welcome also gave her encouragement.

But the doctors said she managed to complete her duties related to the enthronement ceremonies because of her strong sense of responsibility, not because she had fully recovered.

"We believe it's desirable" that she was able to expand her activity, the doctors said in a statement that was also released by the palace.

"But she has not fully recovered and her conditions have ups and downs. She gets tired after a major event or after a series of events," the doctors said. "Having over-expectations could run counter to her recovery."

The doctors said it is important for Empress Masako to continue her treatment while obtaining understanding and support from those around her. "We hope you will continue to warmly watch over her recovery," they said.

There are expectations that Emperor Naruhito - who is Japan's first emperor with a college degree and who studied at Oxford - and Empress Masako will internationalise the imperial household.

Many Japanese were particularly impressed when she and Emperor Naruhito casually chatted with US President Donald Trump and first lady Melania without interpreters during their visit in late May as the first state guests of the new emperor.

The royal couple also freely conversed with many foreign dignitaries who attended state banquets and tea parties to celebrate Emperor Naruhito's enthronement in October.

As a former diplomat, Empress Masako has expressed concerns about global issues, including marine plastic pollution, poverty, child abuse and people in conflict-torn areas.

She mourned the death of Tetsu Nakamura, a Japanese doctor and aid worker who was gunned down in Afghanistan last week.