North Korean leader Kim Jong Un thanked outgoing South Korean president Moon Jae-in for his efforts to improve inter-Korean relations, state media said, in an unexpected move following growing signs Pyongyang is resuming nuclear testing.
North Korea has carried out more than a dozen weapons tests so far this year, and experts have warned of recent signs of new activity at a key nuclear testing site.
Moon's hawkish successor, president-elect Yoon Suk-yeol, has vowed to take a harder line against Kim's provocations.
Seoul's presidential Blue House confirmed Moon and Kim had exchanged friendly letters, but details were not immediately available.
Moon met Kim three times and helped facilitate talks between former US president Donald Trump and the North Korean leader.
But Kim and Trump's nuclear negotiations ended in failure in 2019, due to disagreements on sanctions relief and what North Korea would be willing to give up in return.
Since then, Pyongyang has labeled Moon a "meddlesome mediator," blown up a $15-million (P785.9 million) joint liaison office north of the border that was financed by Seoul, and last month test-fired an ICBM at full range for the first time since 2017.
But on Friday Pyongyang's official KCNA news agency said Kim and Moon agreed that inter-Korean ties would develop if both sides "make tireless efforts with hope."
Kim also said his "historic" summits with Moon gave the people "hope for the future," the KCNA report added.
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 said, adding the duo's letter exchange was an "expression of their deep trust."
Yoon's term begins on May 10.
South Korean officials have said Pyongyang could stage a military parade or carry out a weapons test on or around April 25, the anniversary of the founding of the Korean People's Army. (AFP)