According to North Korea's official media, leader Kim Jong Un got a personal letter from outgoing South Korean President Moon Jae-in on Wednesday and responded on Thursday with his own letter praising Moon's peace efforts throughout his tenure. According to Pyongyang's official Korean Central News Agency, their letter exchange demonstrated their "deep trust".

The announcement comes as North Korea possibly prepares for a nuclear test and other weapons tests.

Just last week, the North Korean leader oversaw the test-firing of a new tactical guided weapon system.

The South Korean Yonhap agency quoted the KCNA as saying that the "test-fire was carried out successfully."

Read more: US envoy travels to Seoul to discuss N.Korea's ICBM launches

According to KCNA, Moon pledged to continue campaigning for reunification after he left office next month and both shared views that “inter-Korean relations would improve and develop as desired and anticipated by the (Korean) nation if the (North and the South) make tireless efforts with hope."

According to Seoul, Moon acknowledged hardships in inter-Korean relations in his letter but maintained that their aspirational pledges for peace during their 2018 summits, as well as an accompanying military agreement aimed at defusing border area confrontations, is a solid foundation for the future of their cooperation.

Moon also voiced optimism for the restart of nuclear discussions between Washington and Pyongyang, as well as for Kim to explore collaboration with Seoul's future administration, which will be led by conservative President-elect Yoon Suk Yeol, according to Moon's Spokesperson Park Kyung-mee.

Yoon, who takes office on May 10, has criticized Moon's foreign policy as "subservient" to North Korea and stated that he will not seek "talks for talks' sake." He has committed to bolstering South Korea's defense in tandem with its partnership with the US, which he claims would involve improving preemptive strike capabilities and anti-missile defense systems to prevent North Korean strikes.

Moon and Kim met several times in 2018, and South Korea aided the US meetings with the North. But the 2019 meeting between Trump and Kim in Vietnam did not go well after the US rejected demands for sanctions relief in exchange for dismantling an old nuclear facility.

The country has been observing a self-imposed embargo on testing long-range and nuclear missiles.

Read more: North Korea: We oppose war, but will retaliate with nukes if attacked

Since then, North Korea has pledged it would increase its nuclear capabilities to counter "gangster-like" US pressure.

North Korea has conducted several missile tests in the last months and will likely continue to do so.