North Korean leader Kim Jong Un thanked the South's outgoing president Moon Jae-in for his efforts to improve inter-Korean relations, Pyongyang and Seoul said Friday, in an unexpected move following signs the isolated state could resume nuclear testing.

The North has carried out more than a dozen weapons tests so far this year, the latest one just last week -- a short-range launch that state media claimed would enhance the "efficiency in the operation of tactical nukes".

The dovish Moon held three summits with Kim 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 labelled Moon a "meddlesome mediator", blown up a $15-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 improve if both sides "make tireless efforts with hope", despite "the difficult situation so far".

In his letter, Kim also said his "historic" summits with Moon gave the people "hope for the future", the KCNA report added.

The North Korean leader "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".

Seoul's presidential Blue House confirmed the two had exchanged friendly letters, adding Moon told Kim that despite the "disappointing moments", together they have made a "clear step" in "changing the fate of the Korean peninsula".

Moon also told Kim in his letter that their inter-Korean summits should become the "foundation for unification", and that he hoped for the swift resumption of the stalled dialogue between North Korea and the United States.

- No substance - 
The letter exchange comes as experts warn of recent signs of new activity at a key nuclear testing site, while Moon's hawkish successor, president-elect Yoon Suk-yeol, has vowed to take a harder line against Kim's provocations. 

Pyongyang's "military threats towards Seoul and frequent rejection of engagement efforts reflect poorly on trust and prospects for cooperation between the two Koreas," Leif-Eric Easley, a professor at Ewha University in Seoul, told AFP.

But "Pyongyang at least needed to acknowledge the end of Moon's presidency," he added, and that the North seems to be "using the occasion to express expectations that Yoon will inherit inter-Korean agreements."

Some analysts said it was "questionable" whether the letter exchange had any substance, given the North's unprecedented blitz of sanctions-busting military provocations this year.

"It would have been meaningful if Moon sent Kim an assertive message that nuclear tests will not be acceptable," Cheong Seong-chang, a senior researcher at the private Sejong Institute, told AFP.

"Kim also sent a warm personal letter to President Moon back in September 2020, but it did not lead to improvement in inter-Korean relations."

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 North Korean army.

The conservative president-elect Yoon's term begins on May 10.