North Korean leader Kim Jong-un has expressed gratitude to outgoing South Korean President Moon Jae-in for his efforts to repair inter-Korean relations, state media reported, in an unexpected move in the wake of rising indications Pyongyang could resume nuclear tests.

On Friday, Pyongyang's official KCNA news agency said that Kim and Moon agreed that inter-Korean relations would improve if both sides made "make tireless efforts with hope."

Kim also stated that his "historic" summits with Moon instilled "hope for the future" in the people, according to the KCNA article.

Kim "appreciated the pains and effort taken by Moon Jae In for the great cause of the nation until the last days of his term of office," the report stated, adding that the duo's letter exchange was an "expression of their deep trust."

According to Seoul's presidential Blue House, Moon and Kim exchanged polite letters, although specifics were not immediately available.

'Interfering mediator'

North Korea has conducted over a dozen weapons tests this year, and experts have warned of recent signals of renewed activity at a critical nuclear testing facility.

President-elect Yoon Suk-yeol, Moon's hawkish successor, has vowed to take a more challenging position against Kim's provocations.

Yoon's tenure officially begins on May 10.

Moon met Kim three times and aided in the facilitation of negotiations between North Korean leader Kim Jong Un and former US President Donald Trump.

However, Kim and Trump's nuclear negotiations collapsed in 2019 because of disputes over sanctions relief and the concessions North Korea would be ready to make in exchange.

Pyongyang has since labeled Moon a "meddlesome mediator," which destroyed a $15 million joint liaison office north of the border financed by Seoul. This month, it conducted its first full-range ICBM test in 2017.

South Korean officials have indicated that Pyongyang may perform a military parade or conduct a weapons test on or around April 25, the anniversary of the Korean People's Army's foundation.