South Korea and North Korea leaders sent 'personal letters' to each other this week

North Korean leader Kim Jong Un responded to a personal letter from outgoing South Korean President Moon Jae-in this week.

According to the North's state news agency KCNA, Moon said he wants to make joint declarations from the two sides "the foundation for the reunification" even after he steps down as president. Kim "appreciated the pains and effort taken" by Moon, the report said.

"The exchange of the personal letters between the top leaders of the North and the South is an expression of their deep trust," KCNA said.

Moon's spokesperson said the president asked Kim to remain committed to cooperation, and that dialogue should overcome the "era of confrontation," Reuters reported.

Kim said "it is our achievement that we signed historic declaration and agreements, which will serve as the guiding post for inter-Korean relations," spokesperson Park Kyung-mi told NBC News.

"President Moon hoped for the resumption of the U.S.-[North Korea] talks at the soonest," she added.

The news comes days after the reclusive North conducted yet another missile test, and weeks before incoming South Korean President Yoon Suk-yeol's inauguration on May 10.

Relations between the two sides initially warmed under Moon, who held three summits with Kim in 2018. But ties deteriorated sharply, and the North threatened military action. It also blew up its joint liaison office with South Korea in 2020.

The letters are a way to sum up the efforts over the last five years, and "the hope is that some aspects of the engagements" between the two sides can carry on, according to Shawn Ho, an associate research fellow at the S. Rajaratnam School of International Studies.

But as Moon's presidential term comes to an end, Ho cautioned, "we shouldn't jump to conclusions that, by extension, this so-called resumption of exchanges between the leaders of the South and the North will lead on to the next administration."

Yoon's government is likely to take a "hugely different" hardline approach with a "totally different set of key players," he told CNBC.

The new administration will want to focus on strengthening cooperation with the U.S. and Japan, he said. "I expect some tensions to rise in the coming months."

On North Korea's side, the letters also do not represent a shift to diplomacy, according to Leif-Eric Easley, a professor at Ewha University in Seoul. He noted that North Korea has planned military displays.

Still, he said the friendly communication between the two sides is "a reminder that inter-Korean relations are not entirely confrontational and should include dialogue."