Japan's royal couple on Thursday listened to the tearful stories of Vietnamese children who were abandoned by their Japanese soldier fathers after WWII, a symbolic meeting in Hanoi aimed at healing wounds between the former war foes.

The 83-year-old Japanese emperor Akihito and his wife, Michiko, are on their first visit to Vietnam, the latest in a series of trips to former battlegrounds.

The elderly couple shook hands and comforted more than a dozen children of the some 700 Japanese military men who decided to stay in Vietnam for a decade after their country's defeat in the second world war.

It was under Akihito's father Hirohito that Japan first sent troops into Vietnam in 1940 when the country was a colony of France.

After the war many of the soldiers stayed on, married Vietnamese women and began raising families as they helped revolutionary leader Ho Chi Minh secure independence from Paris, a little-known chapter of the two countries shared histories.

But they were ordered to leave the country in 1954 and "encouraged" by the Vietnamese government to leave their families behind, said Hatsuhisa Takashima, the emperor's press secretary.

"It might be possible that Vietnamese authorities thought that it was not good for Vietnamese people to jump to a quite new environment," he said.

The half-Japanese children left behind often endured painful ostracisation at home, while their mothers struggled to raise families on their own and were criticised for cosying up to the former occupying forces.

"I understood that families of ex-Japanese soldiers here encountered many difficulties," Emperor Akihito said after listening to tearful stories of separation.

"We are thankful for the attention by the Japanese emperor and empress," said 94-year-old Nguyen Thi Xuan, a former war bride who raised three half-Japanese children on her own and never remarried.

"War brought nothing to us. I only want peace for both countries of Vietnam and Japan," Xuan said.

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Tran Duc Dung, whose Japanese father trained Vietnamese military men during the war, said he was eager to forget the hardships and discrimination his family faced.

"It's time to close all that happened in the past. My only wish now is to be recognised by the Japanese government as a Japanese citizen," Dung told AFP.

Akihito's visit is laden with personal symbolism.

Under Hirohito, considered an emperor god by his followers at the time, Japan embarked on its rapid and often brutal colonial expansion across Asia that still stalks regional relations.

But Vietnam and Japan have built up a warm relationship since diplomatic relations were established in 1973, pushed together by business ties and, increasingly, a mutual suspicion of China.

Japan is now a top aid donor and a leading investor in the communist country. It also welcomes thousands of Vietnamese to its universities.

Akihito and Michiko will fly to Hue the former imperial capital on Friday and then on to Thailand on Sunday.