Trump met South Korean counterpart Lee Jae Myung on Wednesday in the southern town of Gyeongju, where the US leader arrived for a summit of the Asia-Pacific Economic Cooperation (APEC) forum.

On Wednesday, Seoul's presidential aide said the two counries had reached a broad deal covering investment and shipbuilding, while Trump said the agreement was "pretty much" finalised.

"I have given them approval to build a Nuclear Powered Submarine, rather than the old fashioned, and far less nimble, diesel powered Submarines that they have now," Trump said on Truth Social Thursday.

In a separate post, he wrote: "South Korea will be building its Nuclear Powered Submarine in the Philadelphia Shipyards, right here in the good ol' U.S.A."

"Shipbuilding in our Country will soon be making a BIG COMEBACK," he added.

On Wednesday, Lee asked Trump to "make a decision to allow us to receive fuel for nuclear-propelled submarines."

"We are not proposing to build submarines armed with nuclear weapons; rather, diesel submarines have inferior submerged endurance, which limits our ability to track North Korean or Chinese submarines," Lee told Trump.

The dessert at a luncheon for Trump featured the word "PEACE!", according to Seoul's presidential office, echoing the two leaders' first meeting when they pledged to act as a "peacemaker" and a "pacemaker" for peace on the Korean Peninsula.

But tensions with nuclear-armed North Korea remain high after Pyongyang brushed aside Lee's outreach and instead continued deepening military and economic links with Russia.

And Trump said Wednesday he was not able to arrange a meeting with North Korean leader Kim Jong Un during his visit to the South, ending fierce speculation over a possible summit after years of diplomatic deadlock.