Japan's emperor and empress made a detour to Bangkok on Sunday after a six-day trip to Vietnam to pay their last respects to an old friend, the late King Bhumibol Adulyadej.

Emperor Akihito, 83, and his wife Empress Michiko arrived in Bangkok at 1.45 pm.

"The emperor and empress originally planned to visit Vietnam only but because of their friendship and respect to King Bhumibol, they decided to take advantage of their trip to Southeast Asia and make a stopover to Bangkok so they can pay their last tribute to the late king," Mr Hatsuhisa Takashima, press secretary to the emperor for the Thailand trip, told a press briefing on Sunday evening.

"For that purpose, the trip was extended one day to make it possible for them to stay in Bangkok overnight," he added.

From the airport, the imperial couple proceeded to the Mandarin Oriental Hotel where they met with Princess Sirindhorn privately for 30 minutes.

They then proceeded to the Grand Palace to lay a wreath before the remains of Bhumibol, who died on October 13 last year at the age of 88 after reigning for seven decades. Until his death, Bhumibol was the world's longest reigning monarch. His cremation has been tentatively set in late December.

"The emperor and empress have known the late king since the 1960s when they were still the crown prince and princess. This is the last occasion for them to meet the late king and bid farewell," Mr Takashima said.

They signed a condolence book and afterwards met with King Vajiralongkorn at the Amphorn Satharn Villa at Dusit Palace for half an hour.

Mr Takashima said the emperor and empress' separate meetings with King Vajiralongkorn and Princess Sirindhorn were without any translators nor staff around as both families have known each other for a long time.

The last time the imperial couple visited Thailand was in 2006 to attend the 60th anniversary celebrations of Bhumibol's accession to the throne.

In 1990, King Vajiralongkorn, crown prince at the time, attended Akihito's accession ceremony in Tokyo as representative of the Thai royal family. Thailand was the first country Akihito visited after taking the throne.

There have been several exchanges and visits between the two families over their more than 50 years' relations.

Mr Takashima said the Japanese couple appreciated the hospitality extended especially by Thailand's new king, including the use of a cream-coloured Rolls Royce that his late father also used.

"The king himself made several decisions in greeting the emperor and empress in a very personal manner so they will feel comfortable and at home," he said.

In Vietnam, the emperor and empress met with the surviving widows and families of Japanese war veterans.

Akihito's visit was seen as a sign that ties with Japan have improved. Japan invaded Vietnam in 1940 and stayed there until 1945 when it surrendered to the Allied Forces.

"The emperor hopes very much that his visit would contribute to the betterment of Japan-Vietnam relations. He was surely satisfied with the results of the trip," Me Takashima said.

The imperial couple will be staying in Bangkok overnight before flying back to Tokyo on Monday afternoon.