Nearly 400 South Koreans crossed the heavily armed border into North Korea on Tuesday full of joyful anticipation at reuniting with family members separated for more than six decades since the 1950-53 Korean War.

The two Koreas, which are divided and remain in a technical state of war, agreed to hold the family reunions for the first time since February last year after negotiating the end of a standoff at the military border in August.

"I feel really thrilled and happy. I can't wait to see him. I will give him a hug and ask him how he has been doing," said Oh Cheol-hwan, 77, before meeting her 83-year-old brother.

Families separated since the war have no means of communication and often do not know if relatives on the other side are surviving.

In the first round of reunions starting Tuesday, 96 North Koreans and their families will be meeting about 390 people traveling from the South. The second round reunites about 190 North Koreans with 90 South Koreans and their families.

The South Koreans, mostly elderly and some in wheelchairs, gathered in the east coast city of Sokcho near the border on Monday, receiving medical check-ups and a briefing on appropriate conduct while in the North.

On Tuesday morning they boarded buses to cross the border.

The reunions, held in a ballroom of Mount Kumgang resort in the North, are watched by officials and media and include only two hours of private time before ending on Thursday.

South Korean participants are advised to avoid a long list topics such as the North's political leadership or living standards, to the frustration of some of them.

A guidebook suggests preparing notes in advance for subjects they want to talk about with their North Korean kin such as dates when their parents died, as time is limited and emotion can be overwhelming.

Kim Ki-joo is struggling to find what to ask his older brother when they meet for the first time after 65 years separation.

"It is thrilling but I can't organise my thoughts. I can't think of what to say. We are being reunited after 65 years when I was 11. I want to ask if he can recognise me," Kim said.