As South Korea's Moon departs, the North's Kim offers rare praise

North Korean leader Kim Jong Un has thanked South Korea's outgoing president for trying to improve relations in a rare gesture of goodwill.

But analysts said it may not be enough to head off growing tension between the two Koreas.

The warm words from North Korea to President Moon Jae-in came in an exchange of letters less than three weeks before Moon leaves office to be replaced by a conservative leader who has already signalled a tougher line on North Korea.

Analysts were sceptical that North Korea's message heralded a broader improvement in relations, and warned that the praise for Moon could be a bid to portray his successor, Yoon Suk-yeol, as responsible for any further deterioration in ties.

North Korean state media was the first to report the exchange and the unexpected North Korean plaudits for the stalled effort by Moon and his liberal administration to engage.

"Kim Jong Un 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," North Korea's state news agency reported.

The exchange of letters was an "expression of their deep trust", it said.

The letters come against a backdrop of tension since a failed North Korea-US summit in 2019, exacerbated last month when North Korea launched intercontinental ballistic missiles (ICBM), breaking a self-imposed 2017 moratorium.

Moon sent a letter on Wednesday and promised to try to lay a foundation for unification based on joint declarations reached at summits in 2018, despite the "difficult situation", the North's KCNA news agency said.

Moon's office confirmed that he had exchanged "letters of friendship" with Kim.

Moon said the "era of confrontation" should be overcome with dialogue, and that inter-Korean engagement was now a task for the next administration, his spokeswoman, Park Kyung-mee, told a briefing.

Moon also expressed hope for the swift resumption of US-North Korea denuclearisation talks.

North Korea's Kim said in his reply on Thursday that their "historic" summits gave the people "hope for the future", and the two agreed that ties would develop if both sides "make tireless efforts with hope", KCNA reported.

The exchange came as US Special Representative for North Korea Sung Kim was in South Korea for talks.

The US envoy has said he is open to sitting down with the North at any time without preconditions, but it was unclear whether Moon's letter specifically proposed a meeting.

Analysts questioned the North's true intentions.

"This looks more like another step in building the pretext to blame Yoon for more escalation from North Korea, rather than an olive branch to Yoon or Biden," said Markus Garlauskas, a senior fellow with the Atlantic Council think tank and former US national intelligence officer for North Korea.

Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said the letters could signal to Yoon that the door for inter-Korean cooperation was still open, and a potential seventh nuclear test by the North or any other future action would hinge on the new government's approach.

Yoon takes office on May 10. He has said that he is open to dialogue but greater military deterrence and closer ties with the United States are needed to counter the North's "provocations".

Tension escalated when North Korea last month conducted its first full ICBM test since 2017, and there are concerns that it is preparing to restart nuclear testing.