SEOUL, Aug. 24 (Xinhua) -- The ongoing reunions of the war-separated Korean families was in the process of building trust between South Korea and the Democratic People's Republic of Korea (DPRK), a South Korean expert said.

Eighty-one South Koreans, separated by the 1950-53 Korean War from DPRK relatives, left Friday morning for Mount Kumgang in southeast DPRK to meet their long-lost families from the DPRK they never saw since the fratricidal war ended with armistice that left the Korean Peninsula divided.

"The reunions of separated families are in the process of building (inter-Korean) trust. Trust-building is the very first thing to do. Without trust, dialogue is not possible," Lee Sang-man, professor at the Institute for Far Eastern Studies of Kyungnam University in Seoul, said in an interview with Xinhua Thursday.

The first session of reunions was held from Monday to Wednesday at the DPRK's mountain resort, the first in nearly three years. The second session of three-day reunions would last till Sunday. The two Koreas agreed in June to hold the humanitarian event.

It was a follow-up measure to enforce the Panmunjom Declaration, signed by South Korean President Moon Jae-in and top DPRK leader Kim Jong Un after their first summit meeting on April 27 at the border village of Panmunjom. Under the declaration, the two sides agreed to stop all hostile acts and increase exchanges.

Lee said the DPRK's repatriation in July of the remains of the U.S. soldiers killed during the Korean War was also in the process of building trust between Pyongyang and Washington.

The DPRK leader and U.S. President Donald Trump agreed to the repatriation during the first-ever DPRK-U.S. summit in Singapore on June 12.

The repatriation and the family reunions helped build trust among the three parties, and it will help create a good atmosphere for the upcoming inter-Korean summit, Lee forecast. The leaders of the two Koreas agreed to hold their third summit in Pyongyang before the end of September.

"President Moon will visit Pyongyang as a guarantor between North Korea (DPRK) and the United States. Because of the lack of trust (between Pyongyang and Washington), Moon will try to serve as a guarantor, beyond a mediator," said Lee.

The South Korean expert stressed that the peninsula's denuclearization should go in tandem with declaring an end to the Korean War and lifting sanctions toward the DPRK to encourage Pyongyang to keep on the course.

"The U.S. side should make concessions to the North (DPRK) as the United States has colossal tools to pressure the North. But, the North has no such tool. If the United States yields to the North, all will be well," added Lee.