SEOUL, South Korea — Lee Beom-ju, 86, had little to say at first.

“I am sorry, I am sorry,” he told his long-lost younger brother and sister in North Korea when he finally met with them on Thursday, during the first family reunions on the divided Korean Peninsula in more than three years.

Mr. Lee, now a South Korean citizen, fled the North in 1951 during the Korean War. Until Thursday, Mr. Lee had not seen his family since, living with a sense of guilt for failing to look after it as the eldest son, according to pool reports from the South Korean news media. Hwa-ja, the little sister he last saw 63 years ago, is now a 72-year-old grandmother.

“Grandfather told me to run, run and go to the South, away from the war, because I was his eldest grandson,” Mr. Lee said in tears, explaining to his sister and his brother, Yoon-ju, 67, why he had to leave them behind.

Mr. Lee was among 83 older South Koreans, including a 96-year-old grandmother, who crossed the border in buses and ambulances on Thursday to meet with 178 North Korean relatives at the Diamond Mountain resort in southeastern North Korea.