While the neighboring rivals persevere with their efforts toward detente, the U.S. appears increasingly frustrated with a lack of progress since President Donald Trump's historic Singapore summit with Kim in June.

On Friday, Trump said he had directed Secretary of State Mike Pompeo to delay a trip to North Korea, citing insufficient progress on denuclearization. NBC News reported in June that U.S. intelligence agencies believe Pyongyang has increased its production of fuel for nuclear weapons in recent months.

North Korea's state-controlled newspaper accused the United States on Sunday of "double-dealing" and "hatching a criminal plot to unleash a war."

'This is about the future'

During the reunions, family members met in a banquet hall overseen by North Korean minders, exchanging small gifts and sharing meals amid tears and fierce embraces. Because of the economic sanctions placed on the North under the auspices of the United Nations, South Koreans can only bring gifts worth around $100 to offer their relatives.

Chung Hak-soon says she has not forgotten the day that she lost her older brother during the war, only months before the July 1953 armistice ended the violence that had claimed the lives of around 2.5 million civilians.

“The village elder came looking for [him],” Chung said. “He was 16 years old.”

Chung Hak-soon's sister-in-law and nephew wave goodbye to her outside the bus that will take her back to South Korea. It is unlikely she will be permitted to meet them again. Photo courtesy Chung Hak-soon

“My brother asked what he wanted, but the elder just told him to follow,” she said. “That was the last time I ever saw him.”

With the war intensifying near her village and ever more Chinese and North Korean soldiers flooding through the area, the villagers fled to hide in caves. Chung was only 10 years old when she, her mother, and two other siblings fled through the mountains, starving, until they reached U.N. lines where they were greeted by American soldiers.

The fighting soon ended, but life in South Korea was hard at first for refugees from the North. Chung’s mother was haunted by the loss of her son.

“Whenever my mother passed by a young man of his age, she would run after him calling out my brother’s name, hoping that he’d escaped,” Chung said. But it was never him. “Then she would cry her heart out.”

“Her heart was broken until she passed away,” she added.

When this series of reunions was announced, Chung traveled to the office to apply in person.

But when the Red Cross contacted her, the news was bittersweet. Her brother had survived the war, but had passed away years ago. He had left behind a family, however, and Chung would be allowed to meet her brother’s son and widow.

“I asked my sister-in-law how my brother had lived in the North, and she said he’d had a good life,” Chung told NBC News after their meeting earlier this week. “But he died thinking his whole family had been killed in the war, because he couldn’t find us when he came back home.”

“This is about the future,” Chung said. Someday, she hopes, the Koreas will be unified and families will move freely between the two. When that day comes, she wants her and her brother’s descendants to know about their cousins in the North and South.

Time is slipping away for those who lived through the trauma of being torn away from their families during a war whose memory grows increasingly dim for succeeding generations. Tens of thousands South Koreans have applied for reunions, but only a fraction have succeeded in meeting their lost relatives. Over the decades since the war, many have died longing for the places of their birth, wondering if their kin were still alive.

Lee Myung-jin's father was born in North Korea's Wonsan, and told his son he dearly wanted to return to see his birthplace.

So the shipping company executive from Seoul had hoped he would be able to surprise his father with a trip back home. But it didn’t work out, and his father fell ill and passed away soon after.

Photos and documents are Lee Soo-nam's mementos of his life before the Korean War. The small sepia picture to the right was the only image he had of his brother, so he had it retouched and reproduced. David Lom / NBC News

“Before he died, my father asked me to promise that someday I would take him back and bury him in his hometown,” Lee Myung-jin said. “I’ll never forget the promise I made. I still pray that someday I can do it. But I’m getting old, I’m in my 60s now, and I fear the pledge will never be fulfilled in my lifetime.”

On Diamond Mountain, as it was time to end the three day reunion, Lee Soo-nam saw his older brother crying and standing still as the South Koreans walked toward the buses that would carry them away.

He knows it will be nearly impossible to see his brother again: So far, North Korea has never permitted South Koreans to see their relatives more than once during the carefully controlled reunions.

“What can we ever say, to relieve this deep and heavy sorrow?” said Lee Soo-nam. “I can only wish that he will be well.”