Caskets holding what are believed to be the remains of 55 American soldiers who died in North Korea during the war have been returned to the United States, in the first tangible step in implementing a landmark agreement reached between the two countries during their first summit in Singapore.

The transfer yesterday, which coincided with the 65th anniversary of the armistice signed to temporarily end the 1950 to 1953 Korean War and is expected to provide fresh impetus to bilateral talks aimed at dismantling the North's nuclear programme, drew thanks twice in a day from US President Donald Trump.

"After so many years, this will be a great moment for so many families. Thank you to Kim Jong Un," he wrote on Twitter, referring to North Korea's leader.

Hours later, he again praised Mr Kim.

"I want to thank Chairman Kim for keeping his word," Mr Trump said, announcing that Vice-President Mike Pence would meet families of the soldiers when the remains are back in the US.

The White House said in a statement: "We are encouraged by North Korea's actions and the momentum for positive change."

South Korea, too, welcomed the repatriation of war remains, calling it a "humanitarian measure that could help to heal the anguish of those killed in the Korean War and their bereaved families".

Its Ministry of Foreign Affairs said in a statement that the move "marks significant progress that could contribute to the establishment of peace" between North Korea and the US, and the government "expects to see the two sides' efforts accelerated to promote peace and stability on the Korean peninsula".

Mr Kim, whose official title is Chairman of the State Affairs Commission, had agreed to return the war remains in a meeting with Mr Trump in Singapore on June 12.

Mr Kim also agreed to destroy a missile-testing site, as the two leaders pledged to improve ties, build peace and work towards the "complete denuclearisation of the Korean peninsula".

Reports this week said that North Korea has started to dismantle a rocket engine test stand at the Sohae Satellite Launching Station on the west coast, but there has been no official confirmation.

Speaking at a ceremony to mark the return of the war remains at the Osan Airbase in South Korea yesterday, US Forces Korea commander Vincent Brooks said: "It was a successful mission following extensive coordination.

"Now, we will prepare to honour our fallen before they continue on their journey home," he added.

A US military plane flew to the North's eastern port city of Wonsan yesterday morning to receive the remains and take them to Osan Airbase.

General Brooks is slated to host a full honours ceremony for the fallen soldiers next Wednesday, after which the remains will be sent to Hawaii for further processing by the US Defence POW/MIA (Prisoners of War/Missing in Action) Accounting Agency.

About 5,300 US soldiers were believed to have died or gone missing in North Korea during the war.

Over 400 sets of remains have been returned to the US since 1990, when both sides embarked on a project to recover them. The last time North Korea did so was in 2007.

Analysts called the latest repatriation of war remains a positive sign, but warned that the road towards denuclearisation would be fraught with difficulty as both sides cannot agree on how to proceed.

North Korea accused the US of making "regrettable and gangster-like" demands towards rapid denuclearisation, after US Secretary of State Mike Pompeo failed to get any concrete commitment from the regime during talks held in Pyongyang early this month.

The North has also renewed calls for a declaration to end the Korean War, which was halted by an armistice but not replaced with a peace treaty.

The regime argued that such a declaration could add weight to the security guarantee that the US had promised during the June 12 summit in return for denuclearisation. But there are no signs yet if talks regarding the declaration are fruitful.

Dr Shin Beom Chul, senior fellow at the Asan Institute for Policy Studies, told The Straits Times that dialogue has hit a deadlock due to a "big gap" between the two sides.

He noted that Pyongyang prefers a phased approach with early benefits, while the US is demanding sincere, concrete action towards denuclearisation.

"The return of war remains is a good sign of North Korea keeping its promise and wanting to restart dialogue with the US, but I doubt that North Korea's nuclear policy has changed," said Dr Shin.

The two Koreas, meanwhile, are forging ahead with more dialogue and collaboration.

South Korea's Defence Ministry announced yesterday that general-level talks will be held next Tuesday to discuss ways to implement the joint agreement made during the inter-Korea summit in April.

Both sides also held ceremonies yesterday to mark the 65th anniversary of the signing of the armistice.