'Masako is making efforts to perform as many official duties as possible. Please watch her with warmth and take the long-term view,' the prince told a press conference ahead of his week-long visit to Vietnam next week.

Crown Princess Masako, who has skipped most public duties since late 2003 because of stress-induced illness, will not accompany the prince on the trip.

The former career woman has struggled to adjust to life inside the world's oldest monarchy.

The prince said that while the couple understands foreign trips are important to promote Japan's friendship with other countries, the decision to participate in such visits should be made on a case-by-case basis.

'Masako is still under treatment for her illness. It is necessary to make a cautious decision' looking at travel distances, the period of stay and the events to attend, he said.

Prince Naruhito's trip to Vietnam, which starts on Monday, will be his first to a communist nation. Japan is the largest foreign donor to Vietnam.

The prince will make a courtesy call on President Nguyen Minh Triet in Hanoi and visit the ancient city of Hue in central Vietnam, as well as Ho Chi Minh City in the south. -- AFP